证明sina*sinb=[cos(a-b)-cos(a+b)]/2

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证明sina*sinb=[cos(a-b)-cos(a+b)]/2
证明sina*sinb=[cos(a-b)-cos(a+b)]/2

证明sina*sinb=[cos(a-b)-cos(a+b)]/2
cos(a-b)=cosacosb+sinasinb (1)
cos(a+b)=cosacosb-sinasinb (2)
(1)-(2)
cos(a-b)-cos(a+b)=2sinasinb
sinasinb=[cos(a-b)-cos(a+b)]/2

证明:cos(a+b)=cosa×cosb-sina×sinb sinb/sina=cos(a+b),证明3sinb=sin(2a+b) 证明:sin(2a+b)/sinb-2cos(a+b)=sinb/sina急证明sin(2a+b)/sina-2cos(a+b)=sinb/sina 证明sin(2a+b)/sina-2cos(a+b)=sinb/sina (sinA+sinB)/(cos A +cos B) =tan (A+B)/2 怎么证明? 证明sina*sinb=[cos(a-b)-cos(a+b)]/2 如何证明sinA+sinB=2sin((A+B)/2)cos((A-B)/2) sinA(cos(2A+B)+cosB)=cosA(sin(2A+B)-sinB)证明证明下列恒等式：sin(a+b)*cos(a-b）=sina*cosa+sinb*cosb 证明sin(a+b)-sina=2sinb/2*cos(a+b/2) 已知sinb=cos(a+b)sina 证明 tanb=tana/1+2tan^2a 在三角形ABC中,证明2sinA*sinB=-[cos(A+B)-cos(A-B)]从左到右证如何证明在三角形中sinA+sinB+sinC=4coc(A/2)*cos(B/2)*cos(C/2) 如何证明a/sinA=b/sinB=c/sinC=(a+b)/(sinA+sinB) 证明sin(a+b)sin(a-b)=(sina+sinb)(sina-sinb) 求证sina-sinb=2cos(a+b)/2*sin（a-b）/2这个怎么证明? 已知SinA×sinB=1 求cos(A+B)=?