đặt`A=(x+2)(x+3)(x+4)(x+6)-56x²`
`A=(x^2+8x+12)(x^2+7x+12)-56x^2`
`⇔4A=(2x^2+16x+24)(2x^2+14x+24)-224x^2`
`⇔4A=(2x^2+15x+24+x)(2x^2+15x+24-x)-224x^2`
`⇔4A=(2x^2+15x+24)-x^2-224x^2`
`⇔4A=(2x^2+15x+24-15x)(2x^2+15x+24+15x)`
`⇔4A=(2x^2+24)(2x^2+24+30x)`
`⇔A=(x^2+12)(x^2+12+15x)`