已知(tan^2)a=2(tan^2)B+1,求证：(sin^2?
tg^2 A=Sec^2 A-1 Sec^2 A=1/(Cos^2 A) 那么有 Sec^2 A -1 = 2(Sec^2 B -1)+1 Sec^2 A -1 = 2Sec^2 B -1 Sec^2 A =2Sec^2 B Cos^2 B = 2 Cos^2 A 1-Sin^2 B = 2- 2Sin^2 A Sin^2B = 2Sin^2 A -1 搞定