单选题
If A is 25 kilometers east of B, which is 12 kilo-meters south of C, which is 9 kilometers west of D, how far is it, in kilometers, from A to D?
【正确答案】
A
【答案解析】Use TACTIC 1 in Chapter 10: draw a diagram. In the figure below, form rectangle BCDE by drawing DE ⊥ AB. Then, BE = 9, AE = 16, and DE = 12. Finally, DA = 20, because right triangle AED is a 3-4-5 triangle in which each side is multiplied by 4. If you don't realize that, use the Pythagorean theorem to get DA:
(DA)2 = (AE)2 + (DE)2 = 256 + 144 = 400
DA = 20.
(DA)2 = (AE)2 + (DE)2 = 256 + 144 = 400
DA = 20.