Sin²a(sin²a-cos²a)-sin²a=sin²a(sin²a-cos²a-1)=sin²a(sin²a-cos²a-sin²a-cos²a)=
sin²a(-2cos²a)=-1/2sin²2a
-ctga*(-tga)-sin²a=1-sin²a=cos²a
№1
2х² - 20ху + 50у² = 2(х² - 10ху + 25у²) = 2(x - 5y)²
№2
1 )х²у - 36у³ = y(x² - 36y²) = y(x - 6y)(x + 6y)
2) а³ + 8с³ = (a +2c)(a² - 2ac + 4c²)
3) -5m² + 10mn - 5n² = -5 (m² - 2mn + n²) = -5 (m - n)²
№3
1) а^4 - 81 = (a² - 9)(a² + 9) = (a - 3)(a + 3)(a² + 9)
2) 4ас - 28с + 8а - 56 = 4c(a - 7) + 8(a - 7) = (4c + 8)(a - 7) = 4(c + 2)(a - 7)
10,5:5-(3.4+2²*2.5)=10.5:5-(3.4+4*2.5)=10.5:5-(3.4+10)=10.5:5-13.4=2.1-13.4=-11.3