Question

Tìm a:

\(\dfrac{2}{9}.3^{a+1}-4.3^a=-90\)

Answer 1

\(\Rightarrow\dfrac{2\cdot3^{a+1}}{3^2}-4\cdot3^a=-90\\ \Rightarrow2\cdot3^{a-1}-4\cdot3^a=-90\\ \Rightarrow3^{a-1}\left(2-4\cdot3\right)=-90\\ \Rightarrow3^{a-1}=\dfrac{-90}{-10}=9=3^2\\ \Rightarrow a-1=2\Rightarrow a=3\)