已知tanα=-1/2,求2sin^2α+sinα×cosα-3cos^2α的值

问题描述:

已知tanα=-1/2,求2sin^2α+sinα×cosα-3cos^2α的值

原式=(2sin^2a+sinacosa-3cos^2a)/(sin^2a+cos^2a)
=(2tan^2a+tana-3)/(tan^2a+1)
=(2*1/4-1/2-3)/(1/4+1)
=-12/5