Question

If 9^x– 9^x-1 = 72 then the value of 3x = ……………… 

A) 6 

B) – 3 

C) – 6 

D) 4

Answer 1

Correct option is (A) 6

\(9^x-9^{x-1}=72\)

\(\Rightarrow\) \(9^{x-1}(9-1)=72\)

\(\Rightarrow\) \(9^{x-1}=\frac{72}8=9\)

\(\Rightarrow\) x - 1 = 1

\(\Rightarrow\) x = 1 - (-1) = 1+1 = 2

\(\therefore\) 3x = 3 \(\times\) 2 = 6

Answer 2

Correct option is  A) 6