81(x+7)^2-(3x+8)^2 36(x-y)^2-25(2x-1)^2 giúp mình vs . mình cản ơn trc

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`