Question

$frac{1}{2^a} + frac{1}{2^{a-1}}+ frac{1}{2^{a-2}}=28$

Answer 1

$frac{1}{2^{a}}+frac{1}{2^{a-1}}+frac{1}{2^{a-2}} =28\\ frac{1}{2^{a}}+frac{1}{2^{a}cdot 2^{-1}}+frac{1}{2^{a}cdot 2^{-2}} =28\\ frac{1}{2^{a}}+frac{2}{2^{a}}+frac{4}{2^{a}}=28 \\ frac{7}{2^{a}} =28\\2^{a}=frac{7}{28}\\2^{a}=frac{1}{4}\\2^{a}=2^{-2}\\a=-2$