251 1) 2ˣ + 2ˣ⁻³ = 18;

1) $$ 2^x + 2^{x-3} = 18 $$

$$ 2^x + \frac{2^x}{8} = 18 $$

$$ 2^x \left( 1 + \frac{1}{8} \right) = 18 $$

$$ 2^x \cdot \frac{9}{8} = 18 $$

$$ 2^x = 18 \cdot \frac{8}{9} = 2 \cdot 8 = 16 $$

$$ 2^x = 2^4 $$

$$ x = 4 $$

Ответ: 4