Вычислить: sin^2(3π/7)-2tg(1)*ctg(1)+cos^2(-3π/7)+sin^2(5π/2)

Вычислить: sin^2(3π/7)-2tg(1)*ctg(1)+cos^2(-3π/7)+sin^2(5π/2).-----------------sin^2(3π/7)-2tg(1)*ctg(1)+cos^2(-3π/7)+sin^2(5π/2)== (1-cos6π/7)/2 -2+(1+cos( -6π/7)/2 +sin²(2π+π/2) = (1-cos6π/7)/2 -2+(1+cos6π/7)/2 +sin²π/2=(1-cos6π/7+1+cos6π/7)/2 -2+1² =  1 -2+1 = 0.