sin(B/2)=cos(A+B/2) 则B/2+(A+B/2)=90——B+A/2=90 所以为等腰三角形sin(B/2)=cos(A+B/2)则B/2+(A+B/2)=90——B+A/2=90则C+A/2=90——B=C所以为等腰三角形没看明白第一步怎么到第二步的

cos^2a-cos^2b=m,那么sin(a+b)sin(a-b)=? 证明sin(A+B)sin(A-B)=cos^2B-cos^2A 求证cos(a+b)cos(a－b)=cos^2b－sin^2a 求证cos(a+b)cos(a-b)=cos^2a-sin^2b 证明 sin^2A+sin^2B-sin^2A*sin^2B+cos^2A*cos^2证明 sin^2A+sin^2B-sin^2A*sin^2B+cos^2A*cos^2B=1 化简cos(a+b)cos(a-b)+sin^2b sin(a-b)*cos a-cos(a-b)*sin a=1/5,则cos 2b的值是 由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a sin b解题设a为锐角,证：1、2分之根3乘cos a + 2分之1乘sin a=cos(6分之π-a)2、cos a-sin a=根号2cos(4分之π+a) Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2 为什么sin(a+b)-sina=2sin（b/2）cos（a+b/2） 证明三角函数等式sin(A+B)-sinA=2cos(A+B/2)sin(B/2) sin(2A+B)/sinA-cos(A+B)=2,求sin^2(B)+2cos(2A) 已知sin(a+b)sin(a-b)=-1/3,求cos^2a-cos^2b的值 已知sin(a+b)sin(b-a)=m,试求cos^2a-cos^2b的值 证cosA+cos(120+B)+cos(120-B)/(sinB+sin(120+A)-sin(120-A)=tan(A+B)/2 【(sin a+cos a)/(sin a-cos a) 】=3,tan (a-b)=2,tan (b-2a)=? cos(a+b)cos(a-b)+sin(a+b)sin(a-b)=-3/5,求sin(3π/2+2b) 化简sin^2a+sin^2b-sin^2a*sin^2b+cos^2a*cos^2b