Formulas for the Area of Rectangles Triangles and Parallelograms (7)

Establish the formulas for areas of rectangles, triangles and parallelograms and use these in problem solving (ACMMG159)

LO: To find the formula for the area of rectangles, triangles and parallelograms.

Know:

  • the dimensions of 2-D shapes such as length and width (height)
  • the properties of rectangles, triangles and parallelograms.

Understand:

  • That formulas can be used to quickly find the area of a rectangle, triangle or parallelogram.

Do:

  • I can find and use the formula for the area of rectangles, triangles and parallelograms.

Rectangles, Triangles and Parallelograms

Area of Rectangles

Area of Triangles

Area of Parallelograms

Finding the area of rectangles is easy, all you have to do is multiply the length by the width.

Finding the area of triangles is easy, all you have to do is multiply the base by the height and half the answer.

Finding the area of parallelograms is easy, all you have to do is multiply the base by the height.

Another way of finding the area of a rectangle is using an array. An array means using rows and then adding it all up. If each row has 18 squares, if we have 6 rows, we would then go 6 x 18 which is 108 squares in total.

The reason for that is that is if I go base x height (or length x width) that is the same formula as the area of a rectangle. If you look at the picture below each of the triangles is only half of the size of a rectangle, which is why the formula is 1/2 x b x h.

The reason for that is that is if I cut the end piece of the parallelogram, I can slide it to the opposite side and create a rectangle. The area of a parallelogram is the exact same as the area of a rectangle, which is length x width (or base x height).

Teaching Ideas

Area and Perimeter Videos

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