Tìm x, biết: a) x-2/3=-5/21 b) x2 + 15 = 20 c) 2/3x=-5/21 .

a) $x-\frac{2}{3}=-\frac{5}{21}$

$x=\frac{2}{3}-\frac{5}{21}$

$x=\frac{14}{21}-\frac{5}{21}$

$x=\frac{3}{7}$.

Vậy $x=\frac{3}{7}$.

b) x² + 15 = 20

x² = 20 − 15

x² = 5

$x=\sqrt{5}$ hoặc $x=-\sqrt{5}$

Vậy $x\in \{\sqrt{5};-\sqrt{5}\}$.

c) $\frac{2}{3}x=-\frac{5}{21}$

$x=-\frac{5}{21}:\frac{2}{3}$

$x=-\frac{5}{21}.\frac{3}{2}$

$x=-\frac{5}{14}$

Vậy $x=-\frac{5}{14}$.