giúp mình vs ạ đg cần

Câu 1 :

` a , 5x² ( x² - 8x + 11 ) `

` = ` $5x^{4}$ ` - 40x³ + 55x² `

` b , ( 3x + 5 ) ( 2x - 7 ) `

` = 3x ( 2x - 7 ) + 5 ( 2x - 7 ) `

` = 6x²-21x+10x-35 `

 `=6x²-11x-35`

Câu 2 :

` a , x(x-1)+15(x-1)=0`

`⇔(x-1)(x+15)=0`

`⇔ ` \(\left[ \begin{array}{l}x-1=0\\x+15=0\end{array} \right.\) 

`⇔ ` \(\left[ \begin{array}{l}x=1\\x=-15\end{array} \right.\) 

` b , (x-1)² - 81 = 0 `

` ⇔ (x-1)² - 9² = 0 `

` ⇔ ( x-1-9)(x-1+9)=0`

` ⇔ ( x -10)(x+8)=0`

` ⇔ ` \(\left[ \begin{array}{l}x-10=0\\x+8=0\end{array} \right.\) 

` ⇔ ` \(\left[ \begin{array}{l}x=10\\x=-8\end{array} \right.\)