Предмет: Алгебра, автор: 30kostyan

Найти cosA, если 25tg^2(a/2) - 15tg(a/2)-4=0; a принадлежит промежутку: (3p;4p)

Ответы

Автор ответа: mishamish232
0
Угол во 2-й четверти, где синус положителен и вот так вот:
sina=V(1-сos^2(a)0=V(1-24/169=V(144/169)=12/13.
tg2a = sin2a / cos2a = 2sinacosa / [cos^2(a)-sin^2(a]=
2*12/13*(-5/13) / (25/169 - 144/169)=
(-120/169) / (-116/169)=120/116=30/29
вроде правильно
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