数学之美团为你解答
cos(2α) - 2cos(2θ)
= (1 - tan²α) / (1+tan²α) － 2(1 - tan²θ) / (1+tan²θ)
= (1 - tan²α) / (1+tan²α) － 2( - 2 tan²α) / (2+2tan²α)
= (1 - tan²α) / (1+tan²α) + 2tan²α / (1+tan²α)
= (1+tan²α) / (1+tan²α)
= 1