Đáp án:
Hằng đẳng thức:`a^2-b^2=(a-b)(a+b)`
Giải thích các bước giải:
`a, ( 5x - 4 )^2 - 49x^2`
`=(5x-4)^2-(7x)^2`
`=(5x-4-7x)(5x-4+7x)`
`=(-2x-4)(12x-4)`
`=-2(x+2).4(3x-1)`
`=-8(x+2)(3x-1)`
`b, ( 2x + 5 )^2 - ( x - 9 )^2`
`=(2x+5-x+9)(2x+5+x-9)`
`=(x+14)(3x-4)`
`c, ( 3x + 1 )^2 - 4( x- 2 )^2`
`=(3x+1)^2-[2(x-2)]^2`
`=(3x+1)^2-(2x-4)^2`
`=(3x+1-2x+4)(3x+1+2x-4)`
`=(x+5)(5x-3)`