Cos13п/6=cos390=cos30= (корень из 3 )/2
1) y=10-x; x=10-y
2) y=7-2x; x=7-y /2
3) y=x-4; x=y+4
4) x=15+6y; y=x-15 / 6
5) x=5y/4; y=4x/5
6) x=-12-3y /4; y=-12-4x /3
Cos(-5π/3) + cos(π/3) + cos(π)+ cos(-π/4) = 1/2 + 1/2 - 1 + √2/2 = √2/2
(t+2)⋅(t−7)−t2=t^2-7t+2t-14=t^2-5t-14
(-11)^2-5×(-11)-14=121+55-14=162