To simplify the expression \((9 \times 10^{16})^2\), we can apply the following steps:
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Square the coefficient: \[ 9^2 = 81 \]
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Apply the power of a power rule to the exponent: \[ (10^{16})^2 = 10^{16 \times 2} = 10^{32} \]
Now we can combine these results: \[ (9 \times 10^{16})^2 = 81 \times 10^{32} \]
Next, we need to express \(81\) in scientific notation, which is: \[ 81 = 8.1 \times 10^1 \]
Therefore, substituting back into our expression gives: \[ 81 \times 10^{32} = (8.1 \times 10^1) \times 10^{32} = 8.1 \times 10^{1 + 32} = 8.1 \times 10^{33} \]
Thus, the simplified expression in scientific notation is: \[ \boxed{8.1 \times 10^{33}} \]