Решите, . 10 класс. sin^2 (3π/7)-2tg1*ctg1+cos^2(-3π/7)+sin^2(5π/2)

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sin^2 (3π/7)-2tg1*ctg1+cos^2(-3π/7)+sin^2(5π/2) = 0
1) sin^2 (3п/7)+cos^2(-3п/7)= 1
2) -2tg1*ctg1=-2(sint*cost/cost*sint)=-2*1=- 2
3)sin^2(5п/2)= 1
4) 1-2+1=0