QUESTION: The quadrilateral above has four sides, whose lengths are represented by A, B, C, and D.      If we wanted to calculate the area of the quadrilateral, which of the lengths must we know to make the calculation? ANSWERS: The correct answers are in bold.  We must know all four. (Your Answer)   Any three of them. (Correct Answer)   We only need to know A and D.   We only need to know B and D.   None of these answers are right   EXPLANATION: (Note that C does not equal D.) We can see that this shape is a rectangle combined with a right triangle. The rectangle has sides D and C. The triangle has sides equal to C, A, and (B - D). Since it is a right triangle, we could calculate any one side by having the other two sides. (ie. Use C and A to calculate (B - D), or use C and (B - D) to calculate A.) So, if we had any three lengths, we could calculate the other, and therefore, calculate the total area.