□左效平
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平面直角坐標(biāo)系中的對(duì)稱
□左效平
例1(2015·株洲)在平面直角坐標(biāo)系中,點(diǎn)(-3,2)關(guān)于y軸的對(duì)稱點(diǎn)的坐標(biāo)是______.
分析:坐標(biāo)系中單點(diǎn)的對(duì)稱有三種:關(guān)于x軸對(duì)稱,y軸對(duì)稱和原點(diǎn)對(duì)稱.對(duì)于這類(lèi)問(wèn)題的解答,有三種方法:
垂直等距作圖法:
1.過(guò)該點(diǎn)向坐標(biāo)軸引垂線;
2.延長(zhǎng)垂線段,使得延長(zhǎng)線段的長(zhǎng)度等于已知的垂線段的長(zhǎng)度;
3.此時(shí)端點(diǎn)對(duì)應(yīng)的坐標(biāo)即為所求.
SHAN Chan-juan, LONG Jun-rui, WU Bi-bo, QIN Xiao, MEI Chang-lin, WANG Jiu-sheng, XIONG Lin-ping
規(guī)律法:
兩點(diǎn)關(guān)于x軸對(duì)稱:橫坐標(biāo)不變,縱坐標(biāo)互為相反數(shù);兩點(diǎn)關(guān)于y軸對(duì)稱:縱坐標(biāo)不變,橫坐標(biāo)互為相反數(shù);兩點(diǎn)關(guān)于原點(diǎn)對(duì)稱:橫坐標(biāo)與縱坐標(biāo)均互為相反數(shù).
延長(zhǎng)等長(zhǎng)法:
1.連接這個(gè)點(diǎn)與原點(diǎn)之間的線段;
2.延長(zhǎng)線段到新點(diǎn),使得新點(diǎn)到原點(diǎn)的距離等于所連線段長(zhǎng);
3.新點(diǎn)對(duì)應(yīng)的坐標(biāo)即為所求.
解:應(yīng)該填寫(xiě)的答案是(3,2).
例2(2015·南京)在平面直角坐標(biāo)系中,點(diǎn)A的坐標(biāo)是(2,-3),作點(diǎn)A關(guān)于x軸的對(duì)稱點(diǎn),得到點(diǎn)A′,再作點(diǎn)A′關(guān)于y軸的對(duì)稱點(diǎn),得到點(diǎn)A″,則點(diǎn)A″的坐標(biāo)是(____,___).
分析:運(yùn)用規(guī)律法,已知點(diǎn)A的坐標(biāo)是(2,-3),點(diǎn)A′與點(diǎn)A關(guān)于x軸對(duì)稱,得到A′的坐標(biāo)為(2,3),由點(diǎn)A″與點(diǎn)A′關(guān)于y軸對(duì)稱,因此點(diǎn)A″的坐標(biāo)是(-2,3).
解:分別填:-2,3.
例3(2015·涼山)在平面直角坐標(biāo)系中,點(diǎn)P(-3,2)關(guān)于直線y=x對(duì)稱點(diǎn)的坐標(biāo)是().
A.(-3,-2)B.(3,2)
C.(2,-3)D.(3,-2)
分析:解決本題可用到如下兩個(gè)結(jié)論:
1.點(diǎn)A(a,b)關(guān)于直線y=x對(duì)稱的點(diǎn)是B,則B的坐標(biāo)為(b,a).
2.點(diǎn)A(a,b)關(guān)于直線y=-x對(duì)稱的點(diǎn)是B,則B的坐標(biāo)為(-b,-a).
因?yàn)辄c(diǎn)P與點(diǎn)Q關(guān)于y=x對(duì)稱,故點(diǎn)Q的坐標(biāo)為(2,-3),所以選C.
例4(2015·武漢)如下圖,已知點(diǎn)A(-4,2)、B(-1,-2),平行四邊形ABCD的對(duì)角線交于坐標(biāo)原點(diǎn)O.
(1)請(qǐng)直接寫(xiě)出點(diǎn)C、D的坐標(biāo);
(2)寫(xiě)出從線段AB到線段CD的變換過(guò)程;
(3)直接寫(xiě)出平行四邊形ABCD的面積.
分析:(1)根據(jù)點(diǎn)關(guān)于原點(diǎn)的對(duì)稱規(guī)律寫(xiě)出C、D坐標(biāo);
(2)從平移的角度來(lái)說(shuō)明;
(3)點(diǎn)B、C的縱坐標(biāo)相同,故BC∥x軸,同理AD∥x軸.BC長(zhǎng)度可由點(diǎn)B、C的橫坐標(biāo)來(lái)計(jì)算,BC上的高是A、B兩點(diǎn)縱坐標(biāo)的差.
解:(1)C(4,-2)、D(1,2);
(2)AB沿著B(niǎo)C平移BC長(zhǎng)度得到線段CD;
(3)BC=4-(-1)=5,BC上的高為A,B兩點(diǎn)縱坐標(biāo)的差即2-(-2)=4,所以平行四邊形ABCD的面積為5×4=20.
插圖:楊明