已知2sin(3π+a)=cos(π+a)求2sin^2a+3sinacosa-cos^2a的值?
问题描述:
已知2sin(3π+a)=cos(π+a)求2sin^2a+3sinacosa-cos^2a的值?
答
2sin(3π+a)=cos(π+a)
-2sina=-cosa
tana=1/2
2sin^2a+3sinacosa-cos^2a=(2sin^2a+3sinacosa-cos^2a)/(sin^2a+cos^2a)
上下同除cos^2a得(2tan^2a+3tana-1)/(tan^2a+1)
代入tana=1/2
求得=4/5