Rút gọn biểu thức

a) ( x -10 )² - x × ( x + 80)

b) ( 2x + 9 )² - x × ( 4x +31 )

c) 9x² + 42x + 49 - ( 3x +7 )² +405

Anh chị giúp em vs ạ, em, đg cần gấp

`bb\color{Red}\text{Giải:}`

`a)` `( x -10 )^2 - x × ( x+ 80)`

`=x^2-2x×10+10^2-(x^2+80x) `

`=x^2-20x+100-x^2-80x `

`=(x^2-x^2 )+(-20x-80x)+100 `

`=-100x+100 `

`b)` `( 2x+ 9 )^2 - x × ( 4x+31 )`

`=4x^2+2×2x×9+81-(4x^2+31x) `

`=4x^2+36x+81-4x^2-31x `

`=(4x^2-4x^2 )+(36x-31x)+81 `

`=5x+81`

`c)` `9x^2+ 42x+ 49 - ( 3x+7 )^2+405`

`=9x^2+42x+454-(3x+7)^2 `

`=9x^2+42x+454-9x^2-2×3x×7-49 `

`=9x^2+42x+454-9x^2-42x-49 `

`=(9x^2-9x^2 )+(42x-42x)+(454-49) `

`=405`