第07天正弦定理的常见变形及推广高考频度:★★★★☆难易程度:★★★☆☆典例在线(1)已知△ABC中,A=60,3a,则++sin+sin+sinabcABCA.1B.2C.3D.无法求解(2)已知△ABC中,B=45,23b,则其外接圆半径等于A.23B.6C.3D.无法求解【参考答案】(1)B;(2)B.【解题必备】正弦定理的常见变形及推广如下:(1)sinsinsin,,,sinsin,sinsin,sinsinsinsinsinAaCcBbaBbAaCcAbCcBBbAaCc.(2)sinsinsinsinsinsinsinsinsinsinsinsinabcabacbcabcABCABACBCABC.(3)::sin:sin:sinabcABC.(4)正弦定理的推广:2sinsinsinabcRABC,其中R为ABC△外接圆的半径.(5)===2sinsinsinabcRABC的两种变形的应用:①(边化角)2sin,2sin,2sinaRAbRBcRC;②1(角化边)sin,sin,sin222abcABCRRR.熟记正弦定理的变形,可使解题过程更加简捷,从而达到事半功倍的效果.学霸推荐1.在ABC△中,若sincoscosABCabc,则ABC△中最长的边是A.aB.bC.cD.bc或2.在ABC△中,若::ABC1∶2∶3,则a∶b∶c=A.1∶2∶3B.3∶2∶1C.1∶3∶2D.2∶3∶13.已知ABC△的外接圆的半径33Rcm,A=60°,则BC边的长为______________cm.1.【答案】A【解析】由正弦定理可知sincosBB,sincosCC,所以45BC,故90A,所以a为最长的边.故选A.2