Answer

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Hint: Substitute ${3^x}$ to any other variable say (t) then factorize the equation to reach the answer.

The given equation is

${3^{2x}} - 2\left( {{3^{x + 2}}} \right) + 81 = 0$

Now simplify the above equation as ${3^{x + 2}}$ is written as ${3^x} \times {3^2}$ and ${3^{2x}}$ is written as ${\left( {{3^x}} \right)^2}$.

$

{3^{2x}} - 2\left( {{3^x} \times {3^2}} \right) + 81 = 0 \\

\Rightarrow {3^{2x}} - \left( {2 \times 9 \times {3^x}} \right) + 81 = 0 \\

\Rightarrow {\left( {{3^x}} \right)^2} - 18 \times {3^x} + 81 = 0 \\

$

Now let ${3^x} = t...........\left( 1 \right)$, substitute this value in above equation

$ \Rightarrow {t^2} - 18t + 81 = 0$

Now factorize this equation

$

\Rightarrow {t^2} - 9t - 9t + 81 = 0 \\

\Rightarrow t\left( {t - 9} \right) - 9\left( {t - 9} \right) = 0 \\

\Rightarrow \left( {t - 9} \right)\left( {t - 9} \right) = 0 \\

\Rightarrow {\left( {t - 9} \right)^2} = 0 \\

\Rightarrow \left( {t - 9} \right) = 0 \\

\Rightarrow t = 9 \\

$

Now from equation (1)

${3^x} = t = 9$

Now we know 9 is written as ${3^2}$

$ \Rightarrow {3^x} = {3^2}$

So, on comparing

$x = 2$

Hence option (b) is correct.

Note: In such types of questions first simplify the given equation then substitute ${3^x} = t$ this makes the equation simple after this factorizes the equation and calculates the value of t, then re-substitute the value of t and simplifies, we will get the required answer.

The given equation is

${3^{2x}} - 2\left( {{3^{x + 2}}} \right) + 81 = 0$

Now simplify the above equation as ${3^{x + 2}}$ is written as ${3^x} \times {3^2}$ and ${3^{2x}}$ is written as ${\left( {{3^x}} \right)^2}$.

$

{3^{2x}} - 2\left( {{3^x} \times {3^2}} \right) + 81 = 0 \\

\Rightarrow {3^{2x}} - \left( {2 \times 9 \times {3^x}} \right) + 81 = 0 \\

\Rightarrow {\left( {{3^x}} \right)^2} - 18 \times {3^x} + 81 = 0 \\

$

Now let ${3^x} = t...........\left( 1 \right)$, substitute this value in above equation

$ \Rightarrow {t^2} - 18t + 81 = 0$

Now factorize this equation

$

\Rightarrow {t^2} - 9t - 9t + 81 = 0 \\

\Rightarrow t\left( {t - 9} \right) - 9\left( {t - 9} \right) = 0 \\

\Rightarrow \left( {t - 9} \right)\left( {t - 9} \right) = 0 \\

\Rightarrow {\left( {t - 9} \right)^2} = 0 \\

\Rightarrow \left( {t - 9} \right) = 0 \\

\Rightarrow t = 9 \\

$

Now from equation (1)

${3^x} = t = 9$

Now we know 9 is written as ${3^2}$

$ \Rightarrow {3^x} = {3^2}$

So, on comparing

$x = 2$

Hence option (b) is correct.

Note: In such types of questions first simplify the given equation then substitute ${3^x} = t$ this makes the equation simple after this factorizes the equation and calculates the value of t, then re-substitute the value of t and simplifies, we will get the required answer.

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