



 
Hi Eileen. There are many ways to do it. I would start by drawing the triangle, inside the rectangle (remember that the altitude h is perpendicular to the base b). First, you can say that the area of the rectangle is hb. Then you can reason that the triangle's area is ½hb since it neatly divides the rectangle into two pairs of congruent triangles. Now look at the right triangle I've shaded. Can you find an expression for h in terms of a and C? If so, replace h in the ½hb calculation and you will have proved the solution. Cheers,  


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