Problem
20. The length of a rectangular patio is 7 feet and the width is 6 feet. The patio is surrounded by a sidewalk that is \( x \) feet wide. Find the expression that represents the area of the patio, including the side walk.
Solution
Step 1 ：\( A_{patio} = 7 \times 6 \)
Step 2 ：\( A_{total} = (7 + 2x)(6 + 2x) \)
Step 3 ：\( A_{sidewalk} = A_{total} - A_{patio} = (7 + 2x)(6 + 2x) - 7\times6 \)
