a) (15 - x)² = 81
(15 - x)² = 9²
15 - x = 9
x = 15 - 9
x = 6
Vậy x = 6
b) (x - $\frac{1}{3}$)² = 16
(x - $\frac{1}{3}$)² = 4²
x - $\frac{1}{3}$ = 4
x = 4 + $\frac{1}{3}$
x = $\frac{13}{3}$
Vậy x = $\frac{13}{3}$
c) (x + 7)³ = 216
(x + 7)³ = 6³
x + 7 = 6
x = 6 - 7
x = -1
Vậy x = -1