If 9x– 9x-1 = 72 then the value of 3x = ……………… A) 6 B)

1.	If 9x– 9x-1 = 72 then the value of 3x = ……………… A) 6 B) – 3 C) – 6 D) 4
Answer» 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 Correct option is A) 6

If 9x– 9x-1 = 72 then the value of 3x = ……………… A) 6 B) – 3 C) – 6 D) 4

Answer»

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

Correct option is A) 6