Evaluating logarithmic expressions Evaluate each expression. (a) $\log _{7} \frac{1}{49}=\square$ (b) $\log _{3} 81=$

Step 1 ：To solve this, we need to find the power to which 7 must be raised to get 1/49.

Step 2 ：We know that \(7^2 = 49\), so \(7^{-2} = 1/49\).

Step 3 ：\(\log _{7} \frac{1}{49} = -2\)

Step 4 ：\boxed{-2}

Step 5 ：To solve this, we need to find the power to which 3 must be raised to get 81.

Step 6 ：We know that \(3^4 = 81\).

Step 7 ：\(\log _{3} 81 = 4\)

Step 8 ：\boxed{4}