Đáp án:
`65^2 + 15^2 - 20^2 + 65 . 30`
`= (65^2 + 65.30 + 15^2) - 20^2`
`= (65^2 + 2.65.15 + 15^2) - 20^2`
`= (65 + 15)^2 - 20^2`
`= 80^2 - 20^2`
`= (80 + 20)(80 - 20)`
`= 100 . 60`
`= 6000`
Tìm `x`
`x^2 (x - 3) - x + 3 = 0`
`⇔ x^2 (x - 3) - (x - 3) = 0`
`⇔ (x - 3)(x^2 - 1) = 0`
`⇔ (x - 3)(x^2 - 1^2) = 0`
`⇔ (x - 3)(x - 1)(x + 1) = 0`
`⇒` $\left[\begin{matrix} x - 3 = 0\\ x-1=0\\x + 1 = 0\end{matrix}\right.$
`⇒` $\left[\begin{matrix} x = 3\\ x = 1\\x = -1\end{matrix}\right.$
Vậy `x ∈ {3, - 1 , 1}`