Lời giải :
24,
Cách 1 :
`(a-2b)^2-(3a+b)^2`
`=a^2-2.a.2b+(2b)^2-[(3a)^2+2.3a.b+b^2]`
`=a^2-4ab+4b^2-(9a^2+6ab+b^2)`
`=a^2-4ab+4b^2-9a^2-6ab-b^2`
`=(a^2-9a^2)+(-4ab-6ab)+(4b^2-b^2)`
`=-8a^2-10ab+3b^2`
`=-8a^2+2ab-12ab+3b^2`
`=2a(-4a+b)+3b(-4a+b)`
`=(2a+3b)(-4a+b)`
Cách 2 :
`(a-2b)^2-(3a+b)^2`
`=[a-2b+(3a+b)][a-2b-(3a+b)]`
`=(a-2b+3a+b)(a-2b-3a-b)`
`=(4a-b)(-2a-3b)`
`=-(4a-b)(2a+3b)`
`=(-4a+b)(2a+3b)`
