已知tanα=-3 求2cos^2(a)+sixacosa-sin^2a
问题描述：
已知tanα=-3 求2cos^2(a)+sixacosa-sin^2a
答
tan(a)=sin(a)/cos(a)=-3 sin(a)=-3 *cos(a)
2cos^2(a)+sin(a)cos(a)-sin^2(a)=2cos^(2)-3cos^2(a)-9cos^2(a)=-11cos^2(a)
sin^2(a)+cos^2(a)=1 9cos^2(a)+cos^2(a)=1 cos^2(a)=1/10
所以原式=-11/10