`#tnvt`
`(x+3)^2-x^2+15=1`
`<=>x^2+6x+9-x^2+15=1`
`<=>6x=1-15-9`
`<=>6x=-23`
`<=>x=-23/6`
Vậy `x=-23/6`
$\\$
`b)(5-x)^2+6x-x^2=-7`
`<=>25-10x+x^2+6x-x^2=-7`
`<=>-4x=-7-25`
`<=>-4x=-32`
`<=>x=8`
Vậy `x=8`
$\\$
`(1+x)(1-x)-(x-4)^2=-15`
`<=>1^2-x^2-(x^2-8x+16)=-15`
`<=>1-x^2-x^2+8x-16=-15`
`<=>-2x^2+8x=-15+16-1`
`<=>-2x(x-4)=0`
`<=>[(-2x=0),(x-4=0):}`
`<=>[(x=0),(x=4):}`
Vậy `x\in{0;4}`
$\\$
`(x+7)^2-(x+6)(x-6)+6x=21`
`<=>x^2+14x+49-x^2+36+6x=21`
`<=>20x=21-36-49`
`<=>20x=-64`
`<=>x=-16/5`
Vậy `x=-16/5`
