Question

2cos^2x-cos^2x-3=0 решить уравнение

Answer 1

Пусть cosx = t, тогда
2t^2 - t - 3 = 0 
D = 1 + 12*2 = 25
t1  =( 1 + 5)/4 = 6/4 = 3/2 = 1,5 ∉ [ - 1; 1]
t2 = ( 1 - 5)/4 = - 4/4 = - 1 ∈ [ - 1; 1]

cosx = - 1
x = pi + 2pik, k ∈ Z