# 1979 AHSME Problems/Problem 3

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## Problem 3

$[asy] real s=sqrt(3)/2; draw(box((0,0),(1,1))); draw((1+s,0.5)--(1,1)); draw((1+s,0.5)--(1,0)); draw((0,1)--(1+s,0.5)); label("A",(1,1),N); label("B",(1,0),S); label("C",(0,0),W); label("D",(0,1),W); label("E",(1+s,0.5),E); //Credit to TheMaskedMagician for the diagram[/asy]$

In the adjoining figure, $ABCD$ is a square, $ABE$ is an equilateral triangle and point $E$ is outside square $ABCD$. What is the measure of $\measuredangle AED$ in degrees?

$\textbf{(A) }10\qquad \textbf{(B) }12.5\qquad \textbf{(C) }15\qquad \textbf{(D) }20\qquad \textbf{(E) }25$

## Solution

Solution by e_power_pi_times_i

Notice that $\measuredangle DAE = 90^\circ+60^\circ = 150^\circ$ and that $AD = AE$. Then triangle $ADE$ is isosceles, so $\measuredangle AED = \dfrac{180^\circ-150^\circ}{2} = \boxed{\textbf{(C) } 15}$.

## Solution 2 (Obnoxiously tedious)

WLOG, let the side length of the square and the equilateral triangle be $1$. $\angle{DAE}=90^\circ+60^\circ=150^\circ$. Apply the law of cosines then the law of sines, we find that $\angle{AED}=15^\circ$. Select $\boxed{C}$.

Please don't try this solution. This is just obnoxiously tedious and impractical.

~hastapasta

~ pi_is_3.14