sinα=1/6
α∈(pi/2;pi)⇒2 четв.
cosα= - √(36/36-1/36)= - √35/6
18cos2α=18(2cos^2α-1)=18*(2*(-√35/6)^2-1)=18*(2*(35/36)-1)=18*(17/18)=17
C1
1-sin^2x+2sinxcosx=0
sin^2x+cos^2x-sin^2x+2sinxcosx=0
cos^2x+2sinxcosx=0
cosx*(cosx+2sinx)=0
1)
cosx=0
x=pi/2+pik, k∈Z
2)
2sinx+cosx=0 /:cosx≠0
2tgx+1=0
tgx=-1/2
x= - arctg(1/2)+pik, k∈Z
+отбор корней: pi/2
