Find a missing side
From the perimeter, subtract both lengths, then divide by 2. From the area, divide by the side you know.
Questions in this lesson
- 0:20 Today's question: Lin draws a rectangle with perimeter 38 cm. Its length is 12 cm. Find its breadth.
- 1:22 Example 1: Lin draws a rectangle with perimeter 42 cm. Its length is 13 cm. Find its breadth.
- 2:18 Example 2: Lin draws a rectangular panel with area 54 cm². Its breadth is 6 cm. Find its length.
Transcript
You have found the perimeter of a rectangle by adding all four sides. Today, we will work backwards from area or perimeter to find a missing side. Lin draws a rectangle with perimeter 38 centimetres. Its length is 12 centimetres. Find its breadth. A rectangle has 2 lengths, and each is 12 centimetres. If we subtract both lengths from the perimeter, the 2 breadths are left. 38 minus 12 minus 12 is 14, so the 2 breadths make 14 centimetres. The 2 breadths are equal, so divide by 2. 14 divided by 2 is 7 centimetres. Check: 12 plus 7 plus 12 plus 7 is 38 centimetres. If you know the area instead, divide it by the side you know.
Lin draws a rectangle with perimeter 42 centimetres. Its length is 13 centimetres. Find its breadth. Try it first. Pause if you like. A rectangle has 2 lengths, and each is 13 centimetres. If we subtract both lengths from the perimeter, the 2 breadths are left. 42 minus 13 minus 13 is 16, so the 2 breadths make 16 centimetres. The 2 breadths are equal, so divide by 2. 16 divided by 2 is 8 centimetres.
Lin draws a rectangular panel with area 54 square centimetres. Its breadth is 6 centimetres. Find its length. Try it first. Pause if you like. We know the area and the breadth, but the length is missing. Area of a rectangle is length times breadth. The breadth is 6 centimetres, so length times 6 is 54. To undo times 6, we divide by 6. 54 divided by 6 is 9 centimetres. From the perimeter, subtract both lengths, then divide by 2. From the area, divide by the side you know.