解
tana=2
(sin^2a+sinacosa-2cos^2a)/(sin^2a+cos^2a)——除以sin^2a+cos^2a=1,值不变
=(tan^2a+tana-2)/(tan^2a+1)——分子分母除以cos^2a
=(4+2-2)/(4+1)
=4/5
^2这个是平方