$\\$
Bài `1.`
`a,`
`5a (a-2) - a +2`
`= 5a (a-2) - (a-2)`
`= (a-2) (5a-1)`
`b,`
`7 (a-5) + 8a (5-a)`
`= 7 (a-5) - 8a (a-5)`
`= (a-5) (7-8a)`
`c,`
`25a^2 - 4b^2 + 4b - 1`
`= 25a^2 - [4b^2- 4b + 1]`
`= 25a^2 - [(2b)^2 - 2 . 2b . 1 + 1^2]`
`= 25a^2 - (2b - 1)^2`
`= (25a - 2b + 1) (2a + 2b-1)`
$\\$
Bài `2.`
`x^4 + 2x^3 + 2x^2 + 2x + 1=0`
`-> x^4 + x^3 + x^3 + x^2 + x^2 + x +x+1=0`
`-> (x^4+ x^3) + (x^3 + x^2) + (x^2 + x) + (x+1)=0`
`-> x^3 (x+1) + x^2 (x+1) + x (x+1) + (x+1)=0`
`-> (x+1) (x^3 + x^2 + x + 1)=0`
`-> (x+1) [ (x^3 + x^2) + (x+1)]=0`
`-> (x+1) [x^2 (x+1) + (x+1)]=0`
`-> (x+1) [(x+1) (x^2 +1)]=0`
`-> (x+1)^2 (x^2 + 1)=0`
Trường hợp 1 :
`-> (x+1)^2=0`
`->x+1=0`
`->x=-1`
Trường hợp 2 :
`->x^2+1=0`
`->x^2=-1` (Vô lí vì `x^2 ≥0∀x`)
Vậy `x=-1`
