`b, 5(x + 2)^2 < (2x + 3)(2x - 3) + (x - 5)^2 + 30x`
`⇔ 5(x^2 + 4x + 4) < 4x^2 - 9 + x^2 - 10x + 25 + 30x`
`⇔ 5x^2 + 20x + 20 < 5x^2 + 20x + 16`
`⇔ 20 < 16` $\text{(vô lí :D)}$
$\text{Vậy x ∈ ∅}$
`c, \frac{x+1}{2}+\frac{x+2}{3}>\frac{x+3}{4}+\frac{x+4}{5}`
`⇔ \frac{30(x+1)}{60}+\frac{20(x+2)}{60}>\frac{15(x+3)}{60}+\frac{12(x+4)}{60}`
`⇒30(x+1)+20(x+2)>15(x+3)+12(x+4)`
`⇔ 30x + 30 + 20x + 40 > 15x + 45 + 12x + 48`
`⇔ 23x > 23`
`⇔ x > 1`
