Đáp án+Giải thích các bước giải:
`1)`
`(2a+b-5)(2a-b+5)`
`=[2a+(b-5)].[2a-(b-5)]`
`=(2a)^2-(b-5)^2`
`=4a^2-(b^2-10b+25)`
`=4a^2-b^2+10b-25`
`2)`
`(a+1)^2-(a-1)^2-3(a+1)(a-1)`
`=a^2+2a+1-(a^2-2a+1)-3(a^2-1)`
`=a^2+2a+1-a^2+2a-1-3a^2+3`
`=(a^2-a^2-3a^2)+(2a+2a)+(1-1+3)`
`=-3a^2+4a+3`
`3)`
`a^2-4a+a(b-2)-ab+9`
`=a^2-4a+ab-2a-ab+9`
`=a^2+(-4a-2a)+(ab-ab)+9`
`=a^2-6a+9`
`4)`
`(3a-2b)^2-3a(3a-4b)-5b^2`
`=(3a)^2-2.3a.2b+(2b)^2-9a^2+12ab-5b^2`
`=9a^2-12ab+4b^2-9a^2+12ab-5b^2`
`=(9a^2-9a^2)+(-12ab+12ab)+(4b^2-5b^2)`
`=-b^2`
