Question

Tìm n ϵ N, biết 2ⁿ⁺² + 2ⁿ = 20, kết quả là:

A. n = 4

B. n = 1

C. n = 3

D. n = 2

Answer 1

\(2^n\left(2^2+1\right)=20\)

\(\Leftrightarrow2^n.5=20\)

\(\Leftrightarrow2^n=4\)

\(\Leftrightarrow2^n=2^2\)

\(\Leftrightarrow n=2\)

Answer 2

`2^(n+2) +2^n =20`

`<=> 2^n ( 2^2 +1) =20`

`<=> 2^n *5  =20`

`<=> 2^n = 4`

`=> 2^n =2^2`

`=> n=2`

`=>D`