Question 1117399
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The bases of triangles A1 and A3 are two of the parallel sides of the rectangle; call it the length.<br>
The two altitudes of triangles A1 and A3 together are then the width of the rectangle.<br>
Then the sum of the areas of A1 and A3, using the standard formula for the area of a triangle, is one-half the length of the rectangle times the width of the rectangle.<br>
But the area of the rectangle is length times width; therefore the sum of the areas of A1 and A3 is half the area of the rectangle.