Đáp án + Giải thích các bước giải:
Bài `3:`
`(x-5)(x+2)+3(x-2)(x+2)-(3x-1/2)^2+5x^2`
`=x^2+2x-5x-10+3(x^2-4)-[(3x)^2-2.3x. 1/2+(1/2)^2]+5x^2`
`=x^2-3x-10+3x^2-12-(9x^2-3x+1/4)+5x^2`
`= x^2-3x-10+3x^2-12-9x^2+3x-1/4+5x^2`
`=(x^2+3x-9x^2+5x^2)-(3x-3x)-10-12-1/4`
`=-89/4`
`to đpcm`
Bài `4:`
`a)`
`(x+2y^2)^2=x^2+2.x.2y^2+(2y)^2=x^2+4xy^2+4y^4`
`b)`
`(a-5/2)^=a^2-2.a. 5/2 +(5/2)^2=a^2-5ab+25/4b^2`
`c)`
`(m+1/2)^2=m^2+2.m . 1/2+(1/2)^2=m^2+m+1/4`
`d)`
`x^2-16y^4=x^2-(4y^2)^2=(x-4y^2)(x+4y^2)`
`e)`
`25a^2-1/4b=(5a)^2-(1/2b)^2=(5a-1/2b)(5a+1/2b)`
