SinX+cosX=0
sinx+cosx=0
sinx=-cosx
cos(pi/2 - x + 2n pi)=cos(pi+x)
pi/2 - x + 2n pi = pi+x
x=-pi/4 + n pi
interval [0;2pi] n=1, i 2
x=(3/4)pi i x=(7/4)pi