Đáp án+Giải thích các bước giải:
`a) 81(x+7)^2-(3x+8)^2`
`= 81. (x^2 + 2.x.7 + 7^2) - [(3x)^2 + 2.3x.8 + 8^2]`
`= 81 . (x^2 + 14x + 49) - (9x^2 + 48x + 64)`
`= 81x^2 + 1134x + 3969 - 9x^2 - 48x - 64`
`= (81x^2 - 9x^2) + (1134x - 48x) + (3969 - 64)`
`= 72x^2 + 1086x + 3905`
`b) 36(x-y)^2 - 25.(2x-1)^2`
`= 36.(x^2 - 2xy + y^2) - 25[(2x)^2 - 2.2x.1 + 1^2]`
`= 36x^2 - 72xy + 36y^2 - 25.(4x^2 - 4x + 1)`
`= 36x^2 - 72xy + 36y^2 - 100x^2 + 100x - 25`
`= (36x^2 - 100x^2) - 72xy + 36y^2 + 100x - 25`
`= -64x^2 - 72xy + 36y^2 + 100x - 25`
`#SunHee`