Question

решить уравнение 

2sin^2x+5cosx+1=0

Answer 1

sin^2x=1-cos^2x

2(1-cos^2x)+5cosx+1=0

2-2cos^2x+5cosx+1=0

cosx=t

-2t^2+5t+3=0

  2t^2-5t-3=0

 D=25+24=7^2

 t=5+7/4=3

 t2=5-7/4=-1/2

 cosx=-1/2 

x=2pi/3+2pi*k