sina+cosa=0.8
sina*cosa=?
(sina+cosa)^2=(0.8)^2
sin^2(a)+cos^2(a)+2sina*cosa=0.64
1+2sina*cosa=0.64
sina*cosa=(0.64-1)/2=-0.36/2=-0.18
(sin α + cos α)² = sin² + 2 * sin α * cos α + cos²α = 1 + 2 * sin α * cos α = 0,8² = 0,64 ,
поэтому sin α * cos α = (0,64 - 1) / 2 = -0,18
