有1+tan^2a=1/cos^2a
1+1/tan^2a=1/sin^2a
可以求得sin^2a=9/10
cos^2a=1/10
所以原式等于=sin^2a+cos^2a-2cos^2a
=sin^2a-cos^2a=9/10-1/10
=4/5