Question

Let $ABCD$ be an isosceles trapezoid with $\overline{BC} \parallel \overline{AD}$ and $AB = CD.$ Points $X$ and $Y$ lie on diagonal $\overline{AC}$ with $X$ between $A$ and $Y,$ as shown in the figure. Suppose $\angle AXD = \angle BYC = 90^\circ,$ $AX = 3,$ $XY = 1,$ and $YC = 2.$ What is the area of $ABCD?$

$15$

$5\sqrt{11}$

$3\sqrt{35}$

$18$

$7\sqrt{7}$

Answer: C

Concepts:coordinate geometry trapezoid shoelace formula

Difficulty rating: 2170

Solution:

Put $A = (0,0),$ $X = (3,0),$ $Y = (4,0),$ $C = (6,0).$ The right angles give $D = (3, t)$ and $B = (4, s)$ on opposite sides of $AC.$

Parallelism $\overline{AD}\parallel\overline{BC}$ forces $t = -\tfrac{3}{2}s,$ and $AB = CD$ gives $16 + s^2 = 9 + t^2,$ so $t^2 - s^2 = 7.$ Substituting yields $s^2 = \tfrac{28}{5}.$

The shoelace formula gives area $= 3\,|t - s|$ $= 3\cdot\tfrac{5}{2}s$ $= \tfrac{15}{2}s$ $= \tfrac{15}{2}\sqrt{\tfrac{28}{5}}$ $= 3\sqrt{35}.$

Thus, the correct answer is C.

2021 AMC 12A Fall Problem 21

Answer: C

Difficulty rating: 2170

Solution:

Problem 21 in Other Years