§矩阵的乘法及其意义
?平面上的線性變換與二階方陣
主題1,平面上的線性變換,平移、旋轉
,
1.平移運動,設點P(x,y)經平移=(h,k)後得到,因為=+ P'l(',')xy(',')xy(,)xy(,)hk
xhx',,,,,,所以用矩陣
表
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示如下,。 ,,,,,,,,yky',,,,,, I
(2).旋轉運動,
Pxyi'(''),(a)旋轉中心為原點(0,0), , Pxyi(),///P(x+yi)P(x+yi) ) O R
設平面上有一點繞原點O旋轉,角度得到P', (',')xyPxy(,)
令=r,根據三角函數的定義, OP
xr,cos,,可以得知, ,yr,sin,,
,,xrr,,,,cos()(coscossinsin),,,,,,xxy,,cossin,,,,, ,,,,yxy,,sincosyrrsin()(sincoscossin),,,,,,,,,,,,,,
xx',,,,,,,y)寫成如果將P(x,,寫成,則它們之間的關係可寫成 Pxy(,),,,,y'y,,,,
,,cossin,cossin',,,xx,,,,,,,,,,我們稱矩陣A=為旋轉矩陣。 ,,,,,,,,,sin,cos,sincos'yy,,,,,,,,,,
,,cos(,),sin(,),,,1A[討論],旋轉矩陣A的反矩陣= ,,,,sin(,)cos(,),,
xcos()sin()',,,,,x,,,,,,,=。 ,,,,,,,ysin()cos()',,y,,,,,,,,
幾何解釋,
從向量的觀點來看,
,,,,,,,,,
OPOP'若=繞原點O旋轉,成為=, (,)xy(',')xy
,,,,,,,,,cossin',,,xx,,,,,,OPOP'則與滿足。 ,,,,,,,sincos'yy,,,,,,,,
從複數的觀點來看,
P'('')xyi,若在複數平面上P()xyi,繞原點O旋轉,得到,
則=(cos,+isin,) xyi'',()xyi,
(b)旋轉中心為一般的點,
設平面上有一點P繞一點A旋轉,角度得到,那麼與間的關P'(,)xy(,)xy(',')xy(,)xy(',')xy00
,,,,係是什麼,從向量的觀點來看,上述的旋轉運動可以視為=繞原點O旋轉,(,)xxyy,,AP00
,,,,,xxxx,,'cossin,,,,,,,,,00成為=,所以可以得到關係式, ,(,)xxyy,,AP'00,,,,,,yyyy,,'sincos,,,,00,,,,
O,3.旋轉,在坐標平面上一點,若將點以原點為中心依逆時鐘方向旋轉角,得一坐標PPxy(,)
,,,,x,點,則與滿足, Pxy(,)y
,xxcossin,,,cossin,,,,,,,,,,,,而就稱為旋轉矩陣, R[],,,,,,,,,,,,sincosyysincos,,,,,,,,,,,,
,,POX,【說明】設,且與軸正向方的夾角, OPr,OPx
xr,cos,, 則, ,yr,sin,,
,,,,,,POX,,OPOPr,,OP 又且與軸正向方的夾角, x
,xrr,,,,cos()(coscossinsin),,,,,,, 則 ,,yrrsin()(sincoscossin),,,,,,,,,,,
,xxy,,cossin,,, , , ,,yxy,,sincos,,,
由矩陣的乘法運算得知
,xxcossin,,,,,,,,, , ,,,,,,,,yysincos,,,,,,,,
O,【註】若將點以原點為中心,依順時鐘方向旋轉角時,則旋轉矩陣為 Pxy(,)
cos()sin(),,,,,,, , R(),,,,,sin()cos(),,,,,,
4.旋轉矩陣的特性,
cossincossin,,,,,,cos()sin(),,,,,,,,,,,,,11221212,RRR()()(),,,,,,,(1),即, 1212,,,,,,sincossincossin()cos(),,,,,,,,,,1122,,,,,,1212
ncossincossin,,,,,,nn,,,,n,(2),即, [()]()RRn,,,,,,,sincossincosnn,,,,,,,,
※重要範例
,1.在平面上,若點P(1,2)平移(3,2)後得點Q,Q再經過以原點為中心的旋轉30:後得R,則(1) Qd,
的坐標為____________, (2) R的坐標為____________,
33【解答】(1)(4,4);(2)(2, 2,2 , 2)
,【詳解】(1)點P(1,2)經過平移(3,2)的變換得Q,Q的坐標為(1 , 3,2 , 2) , (4,4), d,
(2)Q經過30:旋轉後的位置為R,R的坐標為(x,,y,),
,,31,,,,,4232,,,,,xcos30sin304:,:,,,,,,22,,,, 則, ,,,,,,,,,,,4,,,,sin30cos30::y413223,,,,,,,,,,,,,,,22,,
11,,,102.設A ,,試求A ,, ,,11,,
032,,,【解答】 ,,320,,
11,,,,,,,cossin,,,,,,11,,2244,,【詳解】, A,,,22,,,,1111,,,,,,,,sincos,,,,44,,22,,
1010,,,,,,,,,,cossincossin,,,,01032,,,,,,44221010, A,,(2)32,,32,,,,,,,,101010320,,,,,,,,,,,,sincossincos,,,,4422,,,,
cossin,,,,,,12,3.設,且A ,,若A , I,求之值, 0,,,2,,sincos2,,,,
,,,【解答】0,,, 632
12cossincos12sin1210,,,,,,,,,,,,12【詳解】,所以cos12θ , 1且sin12θ , 0, A,,,,,,,,,sincossin12cos1201,,,,,,,,,,
,,,,因0 , θ ,,所以0 , 12θ , 6π,故12θ , 0,2π,4π,6π,即θ , 0,,,, 2632
cos75sin75cos45sin45:,:::,,,,隨堂練習.,I為單位矩陣,求 A,,,,,sin75cos75sin45cos45::,::,,,,
n(1)使A , I的最小正整數n , ____________,
, 1(2)A , ____________,
,,31,,22,,【解答】(1)12;(2) ,,13,,,22,,
cos75sin75cos45sin45:,:::,,,,【詳解】(1) A,,,,,sin75cos75sin45cos45::,::,,,,cos75cos45sin75sin45cos75sin45sin75cos45::,::::,::,, ,,,sin75cos45cos75sin45sin75sin45cos75cos45::,::::,::,,
cos30sin(30)cos30sin30:,::,:,,,, ,,,,,,sin30cos30sin30cos30::::,,,,
cos(30)sin(30)10:,,:,nn,,,,n ,,?30: , n , 360: , k A,,,,,,sin(30)cos(30)01:,:,nn,,,,
, ,n的最小值為12
cos30sin30:,:,,(2),又行列式值detA , 1 A,,,sin30cos30::,,
,,31,,cos30sin30::,,22,1,,A,, ,, ,,,::sin30cos30,,13,,,,,22,,
01,11,,,,,104., ,若,且,則k = (1) 8 (2) ,8 (3) 16 (4) ,16 (5) 32 Ak,,A,,,,,1011,,,,
【解答】5
11,,,,,11cos45sin45,:,:,,,,22,,A,,,22【詳解】 ,,,,1111sin45cos45::,,,,,,,,22,,
cos450sin450cos90sin9001:,::,:,,,,,,,1010,,?k = 32, A,,,(2)3232,,,,,,sin450cos450sin90cos9010::::,,,,,,
5.設O(0,0),A(4,2)為平面上兩點
(1)若?AOB為正三角形且B在第一象限內,則B的坐標為____________, (2)若OABC為矩形,點C在第二象限內,且,則頂點C的坐標為____________, OCOA,2
33【解答】(1) (2 ,,2, 1);(2) (, 4,8)
【詳解】(1)?AOB , 60:,點B為A經過60:旋轉變換後的位置,設B(x',y'),則
,,13,,,,,423,,xcos60sin604:,:,,,,,,,,22,,,, , ,,,,,,,,,,,2,,,sin60cos60::y231231,,,,,,,,,,,,,,,22,,
33 ?B(2 ,,2, 1),
(2)點A經過90:旋轉後的位置為A'(x,y)時,
xcos90sin9040142:,:,,,,,,,,,,,,,, , ,,,,,,,,,,,,,,,sin90cos9010::y224,,,,,,,,,,,,
A'再經伸縮2倍的位置為C,則C的坐標為(, 4,8),
主題2,平面上的線性變換,鏡射
/P 1.鏡射運動, y 如圖,設平面上有一點P、過原點的直線L的斜角為,, (,)xy
P
, P點對於直線L鏡射的點為,令=r P'OP(',')xy
,
x O x'x=rcos,,y=rsin,,=rcos(2,,,),=rsin(2,,,) y'
x'=rcos(2,,,)=r[cos2,cos,+sin2,sin,]=xcos2,+ysin2,
=rsin(2,,,)=r[sin2,cos,,cos2,sin,]=xsin2,,ycos2, y'
所以可以將與的關係 (,)xy(',')xy
cos2sin2',,xx,,,,,,寫成,, ,,,,,,,,sin2cos2'yy,,,,,,,,
,,cos2sin2,,我們稱為關於L的鏡射矩陣。 ,,sin2,cos2,,,,
,PLP2.鏡射,設,為坐標平面上的相異兩點,其垂直平分線為,則
,,PLPLPP稱與對直線成對稱,同時稱為對於直線的對稱點,而
,PLP對稱點,與直線的關係就像實體,影像與鏡面的關係,因此,
,PLP將平面上一點對應至直線的對稱點之變換就稱為鏡射,其中
L直線稱為鏡射軸,當鏡射軸為一條通過原點的直線時,我們便可以用矩陣來表示這樣的鏡射,
3.用矩陣表示鏡射,
,OLL設直線通過直角坐標的原點且與軸正向方的夾角為,若點對直線的鏡射變換xPxy(,)2
,xxcossin,,cossin,,,,,,,,,,,,,至,則,其中就稱為鏡射矩陣, Pxy(,),,,,,,,,,,,sincosyysincos,,,,,,,,,,,,,
xr,cos,,OPr,OP【說明】設且與x軸逆時鐘方向的夾角為,,則, ,yr,sin,,
,,,2(),,,,OPr,OP 又且與x軸逆時鐘方向的夾角為時, ,,,,2
,xrrr,,,,cos()coscossinsin,,,,,,, , ,,yrrrsin()sincoscossin,,,,,,,,,,,
,xxy,,cossin,,, 得, ,,yxy,,sincos,,,
由矩陣的乘法運算可以得知
,xxcossin,,,,,,,, , ,,,,,,,,yysincos,,,,,,,,,
4.常見的鏡射矩陣,
cos0sin010::,,,,,,,:0(1)鏡射軸為軸時,,即,鏡射矩陣為, ,:0x,,,,,2sin0cos001:,:,,,,,
cos180sin18010::,,,,,,,,:180(2)鏡射軸為軸時,,即,則鏡射矩陣為, ,:90y,,,,,2sin180cos18001:,:,,,,
cos90sin9001::,,,,,,,:90(3)鏡射軸為時,,即。則鏡射矩陣為, ,:45xy,,0,,,,,2sin90cos9010:,:,,,,
cos270sin27001::,,,,,,,,:270(4)鏡射軸為時,,即, 則鏡射矩陣為, ,:135xy,,0,,,,,2sin270cos27010:,:,,,,,
5.鏡射軸為的鏡射矩陣, ymx,
,,,若點對直線L,的鏡射變換到, ymx,Pxy(,)Pxy(,)
2,xx,,12,mm,,,,1則, ,,,,,,,22,yy1,m21mm,,,,,,,
【說明】由三角函數中可以得知,
,,,222cossin1tan,,,,,22222coscos(2)cossin,,,, , ,,,,,,222222cossin1tan,,222
,,,2sincos2tan,,,222sinsin(2),, , ,,,,2sincos2,,,22222,,cossin1tan222
21,m,2mtan,m,,,cos,sin 當,則,, 222,1m1,m
2,,12,mm2,,22,,cossin,,12,mm,,111,,mm,,,, 得鏡射矩陣, ,,,,222sincos,1,m,,21mm,21mm,,,,,,,,22,,11,,mm,,
2,xx,,12,mm,,,,1, 故, ,,,,,,22,yy1,m21mm,,,,,,,
※重要範例
1.設L,x , 2y , 0,點P(4,, 1)對直線L鏡射後得點P'(x',y'),試求P'之坐標為____________,
819【解答】(,) 55
,,1【詳解】已知直線的斜率,所以, m,tan,tan222
,1,12,,2tan2,1tan1432224再由公式求,, sin,,,,,,cos,,11,,55221tan1,,,,1tan12424
348,,,,
,,,,,,cossin44,,,,,,555所以由鏡射變換公式知, ,,,,,,,,,,,,,,,sincos143119,,,,,,,,,,,,,,,,,555,,,,
819 即P'之坐標為(,), 55
2., ,設直線L是有向角30:的終邊所在的直線,點A(3,,4)對直線L鏡射後的坐標為
333333(1) (2) (3) (4) (3,23),,(33,123),(23,23),,(3,13),,222222
33(5) (23,23),,22
【解答】2
11,211,m123m3cos,,,,m,:,tan30,,,【詳解】,, sin22112,m,12m31,3
,,133,,,23,,,,cossin33,,,,,,,,222,,,,,, ,,,,,,,,sincos443,,,,,,,31,,,,,,,,23,,,,,,2,,22,,
33即所求鏡射坐標為, (23,23),,22
隨堂練習., ,坐標平面上點P(6,,4),經直線L,鏡射後的點坐標為 yx,3(1) (2) (3) (4) (233,332),,,(233,332),,,(323,332),,,(233,32),,(5) (233,331),,
【解答】4
,【詳解】L,,, yx,3m,,tan32
21131,,m2233m?, ,cos,,,,,,,,sin221132,,m,,1132m
,,13,,,,,xcossin66323,,,,,,,,,,,,22,,,,,,, ,,,,,,,,,,ysincos44,,,,,,,31233,,,,,,,,,,,,,,,,22,,
即所求鏡射坐標為, (323,233),,,,
3.某點P先以O為中心旋轉80:,再對於直線鏡射,其結果相當P點直(31)(31)0,,,,xy接對於直線y , (tanθ)x鏡射,0: < θ < 180:,求θ之值。 【解答】 155:
31,,m,,,,23tan【詳解】,,,為斜角, (31)(31)0,,,,xy31,
211(743)6433,,,,,m24231m,?, ,,cos,,,,sin,,,22221,m1,m1(743)843,,,1(743),,
,,31,,cos80sin80:,:,,22,,,,鏡射矩陣,又旋轉矩陣, ,,,,,sin80cos80::13,,,,,,,22
,,31,,cos80sin80:,:cos30sin30cos80sin80:::,:,,,,,,22,,? ,,,,,,,sin80cos80::,,sin30cos30sin80cos80:,:::13,,,,,,,,,,,22
cos80cos30sin80sin30sin80cos30cos80sin30::,::,::,::,, ,,,sin30cos80sin80cos30sin80sin30cos80cos30::,::,::,::,,
cos50sin50cos310sin310:,:::,,,,2310,,:,,:155,?,, ,,,,,,,:,::,:sin50cos50sin310cos310,,,,
4., ,下列各選項何者為正確,
,,13,,,,x'x,,,,22 ,,,(1)以原點為中心,點P(x,y)作轉角為120:的旋轉變換得P'(x',y'),則 ,,,,y'y,,31,,,,,,,22,,
34,,,,,x'x,,,, 55(2)點Q(x,y)以直線2x – y = 0為鏡射軸的鏡射變換g得鏡像點Q'(x',y'),則 (3),,,,,,,y'y43,,,,,,
,,55,,點A(x,y)先經以原點為中心,轉角為120:的旋轉變換f再經以直線2x – y = 0為鏡射軸的鏡射變
,,343433,,,,,x'x,,,,1010 ,,換g得點A'(x',y'),則 ,,,,,y'y,,,,,,433343,,,,,,1010,,
(4)點B(x,y)先經以直線2x – y = 0為鏡射軸的鏡射變換g再經以原點為中心,轉角為120:的旋轉
,,343433,,,,,x'x,,,,1010 ,,,變換f得點B'(x',y'),則 ,,,,y'y,,,,,,433343,,,,,,1010,,
(5)點C(x,y)先經以直線2x – y = 0為鏡射軸的鏡射變換g再經此鏡射變換g得點C'(x',y'),則x'x10,,,,,, ,,,,,,,y'y01,,,,,,,
【解答】1234
,,13,,,,x'xxcos120sin120:,:,,,,,,,,22,,,,【詳解】(1),?正確, ,,,,,,,,y'yysin120cos120::,,31,,,,,,,,,,,22,,
21143,,m,244m,cos,,,,(2) L,2x , y = 0,? ,, m,,tan2,,,,sin221145,,m2,,1145m
34,,,,,,,x'xxcossin,,,,,,,,55 ,,?正確, ,,,,,,,,,,,,y'yysincos43,,,,,,,,,,,,,
,,55,,
,,,,34343433,,,13,,,,,,,,,,,x'xx,,,,,,55101022,,,,(3)由(1)(2) ,,?正確, ,, ,,,,,,,,y'yy43,,,,31433343,,,,,,,,,,,,,,,,,,,55,,221010,,,,
,,,,34343433,,,13,,,,,,,,,,,x'xx,,,,,,55101022,,,,(4)由(1)(2) ,,?正確, ,,,,,,,,,,y'yy43,,,,31433343,,,,,,,,,,,,,,,,,,,55,,221010,,,,
x'x10,,,,,,(5)鏡射偶數次會回到原C(x,y),即C' = C ,,?不正確, ,,,,,,,y'y01,,,,,,
5.設O(0,0),A(1,4),且?OAB為等腰三角形,其中?AOB , 45:,且?A , 90:, (1)而B在第一象限內,則B的坐標為____________,
(2)又?AOB對y軸鏡射得?A'OB',而A'為A的鏡射點,B'為B的鏡射點,則A'的坐標為____________,
【解答】 (1)(5,3);(2)(, 1,4)
2【詳解】(1)如下圖,?AOB , 45:,?A , 90:,所以點B為A旋轉, 45:後再伸縮倍,
因此B的坐標(x,y),
,,x20cos(45)sin(45)1,:,,:,,,,,,, ,,,,,,,,sin(45)cos(45),:,:y402,,,,,,,,,,
,,22,,,,201,,1115,,,,,,22,,, ,B(5,3), ,,,,,,,,,,,,4,,,11430222,,,,,,,,,,,,,,,22,,
(2)以y軸為鏡射軸,A的鏡射點A',A'的坐標為(, 1,4),
隨堂練習.設L,x , 2y , 0,L,x , 3y , 0,點P(x,y)對L鏡射後,再對L鏡射,最後得點P'(x',y'),1212
試以x,y表示x',y',
247,,xxy,,,,2525 【解答】,724,,yxy,,,,2525,
1,1,12【詳解】L,x , 2y , 0 ,,L,x , 3y , 0 ,, ,,,,mtanmtan12122232
111,2,21,m2m341412,, cos,,,,sin,,,,112211551,m1,m1111,,44
111,2,21,m2m433922sincos,,,,,,,,, 222211551m,1,m221,1,99
4334,,,,
,,,,xcossincossin,,,,x'x,,,,,,,,,,55552211, ,,,,,,,,,,,,,,,,3443ysincossincos,,y'y,,,,,,,,,,,,,,2211,,,,,,,,,,5555,,,,
247247,,,,xy,,,,,x,,25252525 , ,,,,,,,,724724y,,,,,,,,,xy,,,,25252525,,,,
247724,,?,, xxy,,yxy,,,25252525
6.設L,2x , 6y , 5 , 0,依下列變換,求L'的方程式,
(1)將L以原點為中心,旋轉30:得L',
(2)將L對x ,3y , 0鏡射得L',
3333【解答】(1) (, 3) x , (1 , 3) y , 5 , 0;(2) (1 , 3) x , (, 3) y , 5 , 0
,,31,,,,xxxcos30sin30:,:,,,,,,,,22,,,,【詳解】(1)因, ,,,,,,,,,yyysin30cos30::,,13,,,,,,,,,,22,,
,1,,,,,,,,31313xy,,,,,,,,,,xxx,,,,,,22222,,,,,, 所以, ,,,,,,,,,,,yyy,,,,,,,,13133,,xy,,,,,,,,,,,,,22222,,,,,,
,,,,3xy,,,xy3 將(x,y) , (,)代入L, 22
,,,,3xy,,,xy3 33 得2() , 6() , 5 , 0,即L'為(, 3) x , (1 , 3) y , 5 , 0, 22
1,,,3mtan,(2)因x y , 0的斜率,θ , 60:, 23
,,13,,,xxxcos60sin60::,,,,,,,,22,,,, , ,,,,,,,,,yyysin60cos60:,:,,31,,,,,,,,,,,22,,
,1,,,,,,,,13133xy,,,,,,,,,xxx,,,,,,22222,,,,,, 得,,,, ,,,,,,,,yyy,,,,,,,,31313xy,,,,,,,,,,,,,,,22222,,,,,,
,,,,xy,33xy, 將(x,y) , (,)代入L, 22
,,,,xy,33xy, 33 得2() , 6() , 5 , 0,即得L'為(1 , 3) x , (, 3) y , 5 , 0, 22
7.設L,3x , 5y , 4 , 0,依下列變換求L'的方程式,
(1)將L以O(0,0)為中心旋轉60:而得L',
(2)將L對x , 2y , 0鏡射而得L,,
353,533,【解答】(1) ()x , ()y , 4 , 0;(2) 29x , 3y , 20 , 0 22
【詳解】設P(x,y) , L,P'(x',y') , L',
,,13,,,x'xxcos60sin60:,:,,,,,,,,22,,,,(1) θ = 60: ,, ,,,,,,,,y'yysin60cos60::,,31,,,,,,,,,,22,,
,1,,,,,,131313x'y',,,,,,,,xx'x',,,,,,222222,,,,,, ?, ,,,,,,,,,yy'y',,,,,,313131,,,,,,,,,x'y',,,,,,,,,,,,222222
1331 代入L,3x , 5y , 4 , 0,?, 3()5()40x'y'x'y',,,,,,2222353,533,353,533, ?()x' , ()y' , 4 , 0,即L',()x , ()y , 4 , 0, 2222
213,m24m1,,,,,sincos,,(2) x , 2y = 0,?,,, ,,mtan2255,1m1,m22
34,,
,,,,x'xxcossin,,,,,,,,55 , ,,,,,,,,,,,,y'yysincos43,,,,,,,,,,,,,,,,55,,
,1343434,,,,,,x'y',,,,,,,xx'x',,,,,,555555,,, ?, ,,,,,,,,,,,,yy'y'434343,,,,,,,,,,,,,,,x'y',,,,,,555555,,,,,,
3443 代入L,3x , 5y , 4 , 0,?, 3()5()40x'y'x'y',,,,,5555293 ?,即L',29x , 3y , 20 , 0, x'y',,,4055
1228.求圓C,以直線為鏡射軸經鏡射後的圓方程式, yx,xyxy,,,,,223503
22【解答】 xyx,,,,450
22【詳解】圓C,,其圓心G(1, ),半徑為3, 3(1)(3)9xy,,,,
,,x,,:30鏡射軸的廣義角,圓心G鏡射後的位置(, ), y
,,1,,,,,,,xcos60sin602::則, ,,,,,,,,,,,ysin60cos600:,:3,,,,,,,,,,
2222,C因此圓C鏡射後的圓,其方程式為,即, (2)(0)9xy,,,,xyx,,,,450
主題3,平面上的線性變換,伸縮
1.伸縮運動,設O為平面上一個定點,k為大於0的定數, P
,,,,,,,,,/若將平面上的動點P變換到P,使得=k, OP'OP
則稱此運動為以O為中心的伸縮k倍的運動。
P'
O 2.設P(x,y)經過以原點O為伸縮中心,伸縮k(k>0)倍得到, P'(',')xy
,,,,,,,,,x'因為=k,=k,=kx,=ky OP'OP(',')xy(,)xyy'
kxx0',,,,,,/P與P的關係用矩陣表示如下,。 ,,,,,,,0'kyy,,,,,,
223.伸縮,將單位圓上任一點的縱坐標保持不變, Pxy(,)xy,,1
,,,, 而橫坐標放大為原來的2倍,得一對應坐標Pxy(,)
122,,,,xx,2xx,其中,,即,代入, yy,yy,xy,,12
122,,,,,xy,,1則滿足方程式, Pxy(,)4
其平面坐標上的圖形為一橢圓,像這樣的變換就稱為軸的伸縮變換。 x※也可以對y軸伸縮或同時對軸及y軸作伸縮。 x
4.用矩陣來表示伸縮,
y設與皆為正數,若將坐標平面上任一點的坐標變為倍,坐標變為倍,得一sxsPxy(,)rr
,,,,,xrx,對應點,其中,,我們可以矩陣表示如下, Pxy(,)ysy,
,xrx0r0,,,,,,,,,其中就稱為伸縮矩陣, ,,,,,,,,,,0sysy0,,,,,,,,
r0r0,,,,y【註】伸縮矩陣即將點的x坐標,坐標各伸縮倍,也可以稱將點Pxy(,)Pxy(,)r,,,,0r0r,,,,伸縮倍。 r
5.伸縮與旋轉,
例子,
,以O為中心伸縮2倍以O為中心旋轉60P',,,,,,,,,,,,,,(',')xy點P(x,y)點Q(m,n) 點
,,mx'20xm,,cos60sin60,,,,,,,,,,, y 矩陣表示,,且 ,,,,,,,,,,,,,,,,ny'02ynsin60cos60,,,,,,,,,,/,, PQ
,,20'xx,,cos60sin60,,,,,,,P , ,,,,,,,,,,,02'yysin60cos60,,,,,,,,O x
主題4,平面上的線性變換,推移
1.推移運動,設k是一個常數,在坐標平面上,若將動點P(x,y)的y坐標保持不變,而x坐標
x'變成x+ky,形成P',其中= x+ky,=y,我們稱這種運動為沿x坐標推移y坐標的(',')xyy'
'1kx,,x,,,,k倍。用矩陣表示可為。 ,,,,,,, 01yy',,,, y ,,
2 1倍時, 例如,沿x坐標推移y坐標的31
11,,,xx'xxy',,1,,,,,O 1 x ,, , ,33,,,,,,,yy',,,,,yy',01,,,
這樣的運動將(0,0)、(1,0)、(0,1)、(1,1)、(0,2)、(1,2)
4251依序變成(0,0)、(1,0)、(,1),(,1),(,1),(,2),如右圖所示。 3333
2.設k是一個常數,在坐標平面上,若將動點P(x,y)的x坐標保持不變,而y坐標變成kx+y,
x'P'形成,其中x=,=kx+y,我們稱這種運動為沿y坐標推移x坐標的k倍。用矩(',')xyy'
10'xx,,,,,,陣表示可為。 ,,,,,,,kyy1',,,,,,
OABCOAa,ABb,OABC3.推移,已知一長方形,其長,寬,若將固定,而另一長向右推
,,rbOABCOABCy移單位,,而得一平行四邊形,如圖,,即將矩形上任一點Pay(,)的坐標保
,xary,,,,,,yx持不變,而Pxy(,)坐標得依坐標的倍增加後變換成點,其中,像這樣的變換,r,,yy,,
就稱為推移變換,
4.用矩陣來表示推移,
,,,,,(1)設為一實數,則將平面上任一點對應到,其中,就稱為沿xPxy(,)Pxy(,)xxry,,yy,r
,xrx1,,,,,,軸方向推移坐標的倍,表示如下,, y,r,,,,,,,yy01,,,,,,
,,,,,xx,(2)設為一實數,則將平面上任一點對應到,其中,就稱為沿ysPxy(,)yysx,,Pxy(,)
,xx10,,,,,,軸方向推移坐標的倍,表示如下, , xs,,,,,,,,ysy1,,,,,,
1r10,,,,而二階方陣及皆稱為推移矩陣, ,,,,01s1,,,,
5.推移的特性,
在坐標平面上任一幾何圖形經由推移變換後,其圖形的形狀皆會改變,但面積不變,
,,OABCOABCOAa,ABb,【說明】如前敘中一長方形,其長,寬,經沿軸方向推移至,則長方x
,,OABC,,abOABC,,abOABC形的面積為長寬,而平行四邊形的面積為底高,以長方形,,
,,OABC的面積等於平行四邊形的面積,
※重要範例
,,30,,1.求點A(1, 3),B(, 4)經伸縮矩陣的伸縮變換後位置之坐標, ,21,,0,,2,,
3,6【解答】A變換後的位置之坐標為(3, ),B變換後的位置之坐標為(, 2) 2
,,,,303,,30,,26,,,,,,1,,,,,,【詳解】,, ,,113,,,,,,,,,,,,42300,,,,,,,,,,,,222,,,,,,
3,6故A變換後的位置之坐標為(3, ),B變換後的位置之坐標為(, 2), 2
22..求點A(3,)沿y軸方向推移x軸的倍之後位置的坐標, ,23
,,3【解答】 ,,0,,
,,1033,,,,,,【詳解】, ,2,,,,,,,201,,,,,,3,,
3., ,.坐標平面上一點A(3,,4),若以原點O為旋轉中心,先旋轉60:,再沿x軸方向推移y
5103433,,,,,,5103433坐標的2倍,則新坐標為 (1) (2) (,)(,)2222
56343,,52343,,,,,,,563433(3) (4) (5) (,)(,)(,)222222
【解答】2
【詳解】設新坐標為(x,y)
,,13,,,x12cos60sin603123:,:,,,,,,,,,,,,22,,,,, ,,,,,,,,,,,,y01sin60cos604014::,,,,31,,,,,,,,,,,,,,22,,
,,,,123235103,,,,,,,,3,,222,,,,,, , ,,,4,,,,31433,,,,,,,,222,,,,
,,,,5103433即新坐標為, (,)22
隨堂練習.設P(6,4),依下列的變換,求變換後P'點之坐標, (1)以O為中心旋轉135:,
1(2)以O為中心伸縮倍, 2
(3)對L,( x , (y , 0鏡射, 31),31),
(4)沿y軸推移x坐標的3倍,
33【解答】(1)(, 5,);(2)(3,2);(3)(2 + 3,3 , 2);(4)(6,22) 22
,,22,,,,,,x'cos135sin1356652:,:,,,,,,,,,22 ,,,,,【詳解】(1),?P'(, 5,), 22,,,,,,,,,,y'sin135cos13544::,,222,,,,,,,,,,,,,,,22,,
1,,0,,x'63,,,,,,2 (2),?P'(3,2), ,,,,,,,,,,y'142,,,,,,,,0,,2,,
,31,m,,,,tan23(3), 231,
2211(23)3,,,m24231m, ?, ,,cos,,,sin,,,2222221,m1,m1(23),,1(23),,
,,31,,,,x'6233,,,,,22 ,,,,33 ,,?P'(2 + 3,3 , 2), ,,,,,,y'4,,13323,,,,,,,,,,,,22,,
x'1066,,,,,,,, (4),?P'(6,22), ,,,,,,,,,,y'31422,,,,,,,,
4.在平面上有一定點P作下列各變換後得另一點P'(, 4,3),試求P點之坐標, ,1倍, (4)以原點為中心,縮短為(1)平移向量(1,2), v,2
1(5)沿y軸方向推移x坐標的倍,(2)以原點為中心,旋轉45:, 3
(3)對直線x , 2y , 0鏡射,
,17,24713【解答】(1) P(, 5,1);(2) P(,);(3) P(,);(4) P(, 8,6);(5) P(, 4,) 22535
,x,,415xx1,41x,,,,,,,,,,,,,,,,,,,,【詳解】(1),,,即P(, 5,1), ,,,,,,,,,,,,,,,,,,,,,,,,,,,,32yyy2y 321,,,,,,,,,,,,,,,,,,,,
11,,,
,,,,4xxxcos45sin45:,:,,,,,,,,,,22,,(2), ,,,,,,,,,,,,,311yyysin45cos45::,,,,,,,,,,,,,,22,,
,111,111,,,,,,,
,,,,,,x,4,4,,,,,17,,22222,,,,,, ,,即P(,), ,,,,,,,,,y113,113722,,,,,,,,,,,,,,,,,,22222,,,,,,
1190180:,,:,(3) x , 2y , 0之斜率為,, ,,,,tan22
,213,4cos,,,,sin ,,,,, ,,,sin2,cos25555
34,,,,,,,4xxxcos2sin2,,,,,,,,,,,,55 , ,,,,,,,,,,,,,,,343yyysin2cos2,,,,,,,,,,,,,,,,,,,55,,
,1343424,,,,,,,,,,,,,,,x,4,4,,,,,,,24755555 ,,即P(,), ,,,,,,,,,,,,,,,y433433755,,,,,,,,,,,,,,,,,,,,,,55555,,,,,,
11,,,,00,,,,,xx,4xx2048,,,,,,,,,,,,,,,,,,22(4),,,,即P(, 8,6), ,,,,,,,,,,,,,,,,,,,,,,,,yy131yy0236,,,,,,,,,,,,,,,,,,,,00,,,,2,,2,,
1010,,,,,xx,4x,,,,,,,,,,,,,,(5), 11,,,,,,,,,,,,,3yyy11,,,,,,,,,,,,33,,,,
,110104,,,,,,,x,4,4,,,,,,13,,,,,, ,,即P(, 4,), ,,,1113,,,,,,,,,,,,y331,13,,,,,,,,,,,,333,,,,,,
5., ,下列各方陣所定義的平面變換何者正確, (1)對直線y = 2x鏡射的方陣表示為
34,,,,,0110,,,,55 (2)以原點為中心旋轉90:的方陣表示為 (3)表示在鉛直方向伸張4,,,,,,43,1004,,,,,,,,,55,,
倍,再對x軸做鏡射 (4)若坐標平面上有一正方形面積為a,則在(x,y) ? (x + 3y,y)的變換下,此正
1方形變換成平行四邊形,其面積仍為a (5)若?ABC在(x,y) ? (2x,y)的變換下,變換成?A'B'C',2則?ABC與?A'B'C'的面積相等
【解答】1345
34= 2 , cos2θ =,sin2θ =, 【詳解】(1) tanθ ,55
,34,,
,,,,cos2sin2,,55 ?對y = 2x之鏡射方陣為,?正確, ,,,,,sin2cos243,,,,,,,
,,55,,
cos90sin9001:,:,,,,,(2)以原點為中心旋轉90:之方陣為,?不正確, ,,,,,sin90cos9010::,,,,
101010,,,,,,(3),即鉛直方向伸張4倍,再對x軸做鏡射,?正確, ,,,,,,,040104,,,,,,,,
(4) (x,y) ? (x + 3y,y)表原圖形做水平推移3y此變換面積不變,?正確,
1(5)令A(x,y),B(x,y),C(x,y)作(x,y) ? (2x,y)的變換後, 1122332
111,,, ,,, Axy(2,)Bxy(2,)Cxy(2,)112233222
1122xxyy,,2121xxyy,,111212122 ,,,,2||,|| ?A'B'C'面積?ABC面積, 11xxyy,,222313122xxyy,,313122
?正確,
10,,隨堂練習., ,有關二階方陣所對應的平面變換,下列敘述何者為真, (1)為對,,01,,,
01,10,,,,y軸鏡射的變換 (2)為對直線x = y鏡射的變換 (3)為對原點作對稱的變換 ,,,,01,10,,,,
20,,14,,,,(4)?ABC經作伸縮變換後,面積保持不變 (5)?ABC經方陣作推移變換後,面積保1,,,,001,,,,2,,
持不變
【解答】2345
x'xx10,,,,,,,, 【詳解】(1), P'(x',y') = (x,,y),?對稱x軸,不正確, ,,,,,,,,,,y'yy01,,,,,,,,,,
x'xy01,,,,,,,, (2), P'(x',y') = (y,x),?對稱x = y ,正確, ,,,,,,,,,,y'yx10,,,,,,,,
x'xx,,10,,,,,,,, (3), P'(x',y') = (,x,,y),?對稱原點,正確, ,,,,,,,,,,y'yy01,,,,,,,,,,
20
||1,(4)?,?面積不變,正確, 102
14(5)?,?面積不變,正確, ||1,01
12,, 6.設直線L在方陣的推移變換下,得一新直線L',若L之方程式為4x , 3y , 5 , 0,則L'之,,01,,
方程式為____________,
【解答】4x , 5y , 5 , 0
,,,,xxxy122,xxy,,2xxy,,2,,,,,,,,,,【詳解】,, ,,,,,,,,,,,,,,,yy,yyy01yy,,,,,,,,,,,代入4x , 3y , 5 , 0 , 4(x' , 2y') , 3y' , 5 , 0 , 4x' , 5y' , 5 , 0,
即L'之方程式為4x , 5y , 5 , 0,
7.設平面上有一直線L,4x , 3y , 5,
(1)若將直線L,以原點為中心,作轉角為, 30:的旋轉變換,得一新直線L',則L'之方程式為____________,
(2)若將直線L,對直線2x , y , 0作鏡射變換,得一新直線L',則L'之方程式為____________, (3)若將直線L,沿x坐標方向推移y坐標的2倍,沿y坐標方向推移x坐標的, 3倍,得一新直線
L',則L'之方程式為____________,
33【解答】(1) (4, 3)x , (4 , 3)y , 10;(2) 24x , 7y , 25 , 0;(3) 5x , 11y , 35 , 0
,xxcos(30)sin(30),:,,:,,,,,, 【詳解】(1)設L'上之動點P'(x',y') , ,,,,,,,,yysin(30)cos(30),:,:,,,,,,
,,,,,,313xy,,1,,,,,,,xxxcos(30)sin(30),:,,:,,,,,,,,222,,,,,, ,, ,,,,,,,,,,y,,,,,yysin(30)cos(30),:,:,133xy',,,,,,,,,,,,,222,,,,
,,,,33xyxy,,, 3y , 5 , 由4x 435,,,,22
33,333 , (4)x' , (4 , 3)y' , 10,即L',(4, 3)x , (4 + 3) y , 10,
,(2) θ為直線2x , y , 0之斜角, tanθ , 2,0 , θ , 2
1234 , cosθ ,,sinθ ,, cos2θ ,,sin2θ ,, ,5555
,,3434,,xy,,,,,,,,,,,xxxxcos2sin2,,,,,,,,,,,,555 仿(1),代入L, ,,,,,,,,,,,,,,,,,,,,,yyxy4343,yysin2cos2,,,,,,,,,,,,,,,,,
,,,,555,,,,
,,,,,,,3443xyxy , , 24x' , 7y' , 25 , 24x' , 7y' , 25 , 0, 435,,,,55
即L',24x , 7y , 25 , 0,
1212,,,,,1,1(3)按題意,矩陣A ,, A , ,,,,31,317,,,,
,,xy,2,,
,,,xx12,,,,,,,17 ,代入L, ,,,,,,,,,,,,,yyxy313,7,,,,,,,,
,,7,,
,,,,xyxy,,23 , , 5x' , 11y' , 35 , 5x' , 11y' , 35 , 0, 435,,,,77
即L',5x , 11y , 35 , 0,
228.設圓C,x , y , 9,以O為中心伸縮5倍,求變換後的圖形方程式,
22【解答】x , y , 225
【詳解】設P'(x',y')為變換後圖形上的動點
x'xx505,,,,,,,,11,,?,, xx',,,yy',,,,,,,,,y'yy05555,,,,,,,,
112222代入x + y = 9,? ()()9x'y',,55
2 222, x' + y' = 25 , 9 = 225,即所求方程式為x + y = 22
9.設A為平面上的線性變換,試寫出滿足下列各條件的方陣A, (1)若A將(1, 0),(0, 1)分別映射至(3, 4),(5, 6),
(2)若A將(1, 0),(0, 1)分別映射至(, 2),(3,), ,4,1
,,35,,,13【解答】(1);(2) ,,,,4624,,,,,
,,,,13,,,,05,,35A,A,A,【詳解】(1),,即, ,,,,,,,,,,041646,,,,,,,,,,
,,,,03,,,13,,,,11,A,A,A,(2),,即, ,,,,,,,,,,14,24,02,,,,,,,,,,
,,1aA,隨堂練習.設線性變換,將點(1, 2)映射至點(, 7),點(,)映射至點(c, d),求c,d之,2,1,1,,b2,,
值,
c,,1d,,8【解答】,
,,,,,,,,11121aa,,a,,1b,3,,【詳解】,故,, ,,,,,,,,bb2247,,,,,,,,,
,,,,,,1121,,,,故c,,1,d,,8, ,,,,,,,3218,,,,,,,,
,,13A,隨堂練習.設,求直線L,經A變換後圖形的方程式, 230xy,,,,,,12,,
【解答】 37150xy,,,
【詳解】P(, 1),Q(2, 7)在直線L上, ,1
,,,,,,1312,,,,,,,13223,,,, ,,,,,,,,,,,,,1213,12712,,,,,,,,,,,,過(2, 3)與(23, 12)的直線方程式為, 37150xy,,,故直線L變換後圖形的方程式為, 37150xy,,,
,,12D,10.設A(1, 2),B(, 3),若線段經線性轉換之後的圖形長度, AB,2,,43,,【解答】 82
,,,,,,1215,,,,,,1224,,,【詳解】,, ,,,,,,,,,,,,432104331,,,,,,,,,,,,
22,,,,(54)(101)82,,,,ABA,B變換後得(5, 10),(4, 1),線段之長, AB