2*sin(60°-B)*sinB=cos(60°-2B)-cos60°

2*sin(60°-B)*sinB=cos(60°-2B)-cos60° 求证：sin(a+b)cosa-(1/2)[sin(2a+b)-sinb]=sinb Sin（120°-B/2）+Sin（60°-B/2）=2SinB,求SinB最后答案等于8分之根号39或者：根号3COS B/2=2SIN B 我求到这了 这个表达式是怎么成立的?sin(180-3B)/sinB=sin3B/sinB=[3sinB -4(sinB)^3]/sinB=3-4(sinB)^2 sinb/sina=cos(a+b),证明3sinb=sin(2a+b) f(B)=2SinB*SinB+Sin(2B+30^)等于 证明:sin(2a+b)/sinb-2cos(a+b)=sinb/sina急 证明sina sin(a+2b) - sinb sin(b+2a) =sin(a+b)sin(a-b) 求证：cosA°sinB°=1/2【sin（A+B）°-sin（A－B）°】 急 sin^A=sin^B=sinC(sinB+sinC)三角形ABC中,若sin^A=sin^B=sinC(sinB+sinC),则角A=呃……是sin^2A=sin^2B+sinC(sinB+sinC) sinA=1/2,sinB=1/3,sin(A+B)sin(A-B)=? 求证cosa*sinb=1/2[sin(a+b)-sin(a-b)] 三角形中,已知A=120° ,B+C=60° ,sinB+sinC=1 怎么得到sin B＝sin C＝1/2三角形中,已知A=120°,B+C=60°,sinB+sinC=1怎么得到sin B＝sin C＝1/2我们教科书里没和差化公式. (sinA+sinB)(sinA-sinB) = 1/2 sinC^2是怎么化简成为sin(A+B)sin(A-B) = 1/2sin(A+B)^2 已知A、B为锐角,且sinA*sinA+sinB*sinB=sin(A+B),求证A+B=∏/2 在△ABC中,求证sin(A+B)/(sinA+sinB)+sin(B+C)/(sinB+sinC)+sin(C+A)/(sinC+sinA)>=3/2 证明sin(a+b)sin(a-b)=(sina+sinb)(sina-sinb) 为什么(sinA+sinB)(sinA-sinB)=sin(A+B)*sin(A-B)