若tanAtanB=tanA+tanB=1,cos(A+B)=( )
问题描述:
若tanAtanB=tanA+tanB=1,cos(A+B)=( )
打错了!应该是tanAtanB=tanA+tanB+1
答
由已知得(sinA/cosA)(sinA/cosB)=sinA/cosA+sinB/cosB+1化简得sinAsinB=sinAcosB+sinBcosA+cosAcosB而cos(A+B)=cosAcosB-sinAsinB代入上式得cos(A+B)=sinAcosB+sinBcosA=sin(A+B)又cos(A+B)的平方+sin(...