平方有1+2sinα*cosα = 1/25 ==> sinα*cosα = -12/25;
sina-cosa > 0(α为钝角),平方有1-2sinα*cosα = 49/25,sina-cosa=7/5;
sinα+cosα=1/5; sina-cosa=7/5
sina=4/5,cosa=-3/5,tana=-4/3;
1/cos²α-sin²α = (cos²α+sin²α)/cos²α-sin²α = 1+tan²α-sin²α = cos²α + tan²α = 1/(1/ cos²α) + tan²α = 1/(tan²α+1)+tan²α 带入tana计算