( X^2 + Y^2 + 2XY ) / ( 3Y + 3X ) = [ ( X + Y)*( X + Y ) ] / [ 3 * ( Y + X ) ] =
= ( X + Y ) / 3 = 1/3X + 1/3Y
Cos 4x + cos 2x + √3*cos x = 0
1) cos x = 0;
x1 = pi/2 + pi*k
2) cos 3x = -√3/2
3x = +-11pi/6 + 2pi*n
x2 = +-11pi/18 + 2pi/3*n
79 * 81 = (80-1) (80+1) = 80² - 1² = 6400 - 1 = 6399