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Page 144: Area by Decomposition

April 10, 2024
1 min read

Figure It Out

Task: Find the areas of the figures on the grid by dividing them into rectangles and triangles.

(Note: Grid squares are assumed to be 1 unit).

Figure a (Parallelogram-like):

  • Base = 4 units. Height = 6 units.
  • Area = 4×6=244 \times 6 = 24 sq units.

Figure b (Trapezoid/Triangle combo):

  • Decompose into a central rectangle and side triangles.
  • Solution says: 30 sq units.

Figure c (Complex Polygon):

  • Solution says: 48 sq units.

Figure d (M-shape/Polygon):

  • Solution says: 16 sq units.

Figure e (Diamond/Kite):

  • Solution says: 12 sq units.
  • Calculation: Box it in a rectangle of 4×6=244 \times 6 = 24. The diamond usually takes up half the area of the bounding box if vertices touch the midpoints/sides. 24/2=1224 / 2 = 12.
Tip

Method: To solve these on grid paper:

  1. Draw a bounding rectangle around the shape.
  2. Calculate the area of the bounding rectangle.
  3. Calculate the areas of the empty triangles in the corners (using 12×b×h\frac{1}{2} \times b \times h).
  4. Subtract the corner areas from the bounding box area.