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If $\log _{10} 7=0.8451$, then the position of the first significant figure of $7^{-20}$ is
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Verified Answer
The correct answer is:
17
We have,
$$
\begin{aligned}
&\qquad \log _{10} 7=0.8451 \\
&\text { Let } \quad x=7^{-20} \\
&\Rightarrow \log x=-20 \log 7=-20 \times 0.8451=-16.902 \\
&\Rightarrow x=10^{-16.902}
\end{aligned}
$$
Now, $-16.902$ lies between $-16$ and $-17$
So, position of first significant figure of $7^{-20}$ is 17 .
$$
\begin{aligned}
&\qquad \log _{10} 7=0.8451 \\
&\text { Let } \quad x=7^{-20} \\
&\Rightarrow \log x=-20 \log 7=-20 \times 0.8451=-16.902 \\
&\Rightarrow x=10^{-16.902}
\end{aligned}
$$
Now, $-16.902$ lies between $-16$ and $-17$
So, position of first significant figure of $7^{-20}$ is 17 .
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