Perimeter of a rectangleFinding the perimeter of a rectangle when the length and the width are given and finding the length or the width when the perimeter is given are the goals of this lessonP = l + w + l + w = 2 × l + 2 × w Example #1: Find the perimeter of a rectangle when l = 5 cm and w = 3 cm P = 2 × 5 + 2 × 3 P = 10 + 6 P = 16 Notice that it is perfectly ok to do P = 5 + 3 + 5 + 3 = 8 + 8 = 16 However, it is usually easier and quicker to multiply each dimenssion by 2 and adding the result. Example #2: Find the perimeter of a rectangle when l = 9 cm and w = 5 cm P = 2 × 9 + 2 × 5 P = 18 + 10 P = 28 Example #3: Find the perimeter of a rectangle when l = 2/4 cm and w = 3/6 cm P = 2 × 2/4 + 2 × 3/6 P = 2/1 × 2/4 + 2/1 × 3/6 P = (2 × 2)/(1 × 4) + (2 × 3)/(1 × 6) P = 4/4 + 6/6 P = 1 + 1 = 2 cm Example #4: A rectangle has a perimeter of 14 inches. Find the length of the rectangle if the width is 4 inches Here, given the perimeter and the width, you are asked to find the length of the rectangle. We know that P = 2 × l + 2 × w You should replace P by 14 and w by 4 because that is what they gave you. So, 14 = 2 × l + 2 × 4 14 = 2 × l + 8 The problem becomes a two step equations that you need to solve However, you can solve this equation with mental math. Just say to yourself, " 8 plus what equal 14? " Since 8 + 6 = 14, 2 × l must equal 6 Ask yourself now, " 2 times what equal 6? " Since 2 times 3 is 6, l = 3 Example #5: A rectangle has a perimeter of 18 inches. Find the width of the rectangle if l is 5 inches Here, given the perimeter and the length, you are asked to find the width of the rectangle. We know that P = 2 × l + 2 × w You should replace P by 18 and l by 5 because that is what they gave you. So, 18 = 2 × 5 + 2 × w 18 = 10 + 2 × w Again, the problem is a two step equations that you need to solve However, you can solve this equation with mental math. Just ask yourself, " 10 plus what equal 18? " Since 10 + 8 = 18, 2 × w must equal 8 Ask yourself now, " 2 times what equal 8? " Since 2 times 4 is 8, w = 4 




