Найти 50cos²α, если tgα=7

Question

Найти 50cos²α, если tgα=7

Answer 1

1+ tg ^2 x = 1/ cos^2 x
1+7 = 1/ 50cos^ x 
8= 1/ 50 cos^ x
8* 50 cos^2 x =1
400 cos^ x =1
cos^2 x = 1/400
cosx =1/20
cos x = 0.05