`a)5x^2-5x(x-2)+30=0`
`<=>5x^2-5x^2+10x+30=0`
`<=>10x=-30`
`<=>x=-3`
Vậy `S={-3}`
`b)5x(12x+7)-(3x+1)(20x-5)=-100`
`<=>60x^2+35x-60x^2+15x-20x+5=-100`
`<=>(60x^2-60x^2)+(35x+15x-20x)=-100-5`
`<=>30x=9-105`
`<=>x=-105/30`
`<=>x=-7/2`
Vậy `S={-7/2}`
`c)(x+3)(x-3)-x(x-2)=0`
`<=>x^2-9-x^2+2x=0`
`<=>(x^2-x^2)+2x=9`
`<=>2x=9`
`<=>x=9/2`
Vậy `S={9/2}`
`d)(x+5)^2+(x+4)(4-x)=0`
`<=>x^2+10x+25+16-x^2=0`
`<=>(x^2-x^2)+10x=-25-16`
`<=>10x=-41`
`<=>x=-41/10`
Vậy `S={-41/10}`