2015 AMC 10B Problems/Problem 19


In $\triangle{ABC}$, $\angle{C} = 90^{\circ}$ and $AB = 12$. Squares $ABXY$ and $ACWZ$ are constructed outside of the triangle. The points $X, Y, Z$, and $W$ lie on a circle. What is the perimeter of the triangle?

$\textbf{(A) }12+9\sqrt{3}\qquad\textbf{(B) }18+6\sqrt{3}\qquad\textbf{(C) }12+12\sqrt{2}\qquad\textbf{(D) }30\qquad\textbf{(E) }32$

Solution 1

The center of the circle lies on the perpendicular bisectors of both chords $ZW$ and $YX$. Therefore we know the center of the circle must also be the midpoint of the hypotenuse. Let this point be $O$. Draw perpendiculars to $ZW$ and $YX$ from $O$, and connect $OZ$ and $OY$. $OY^2=6^2+12^2=180$. Let $AC=a$ and $BC=b$. Then $\left(\dfrac{a}{2}\right)^2+\left(a+\dfrac{b}{2}\right)^2=OZ^2=OY^2=180$. Simplifying this gives $\dfrac{a^2}{4}+\dfrac{b^2}{4}+a^2+ab=180$. But by Pythagorean Theorem on $\triangle ABC$, we know $a^2+b^2=144$, because $AB=12$. Thus $\dfrac{a^2}{4}+\dfrac{b^2}{4}=\dfrac{144}{4}=36$. So our equation simplifies further to $a^2+ab=144$. However $a^2+b^2=144$, so $a^2+ab=a^2+b^2$, which means $ab=b^2$, or $a=b$. Aha! This means $\triangle ABC$ is just an isosceles right triangle, so $AC=BC=\dfrac{12}{\sqrt{2}}=6\sqrt{2}$, and thus the perimeter is $\boxed{\textbf{(C)}\ 12+12\sqrt{2}}$. [asy]   /* Geogebra to Asymptote conversion, documentation at artofproblemsolving.com/Wiki, go to User:Azjps/geogebra */ import graph; size(11.5cm);  real labelscalefactor = 0.5; /* changes label-to-point distance */ pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */  pen dotstyle = black; /* point style */  real xmin = -4.3, xmax = 18.7, ymin = -5.26, ymax = 6.3;  /* image dimensions */   draw((3.46,0.96)--(3.44,-3.36)--(8.02,-3.44)--cycle);  draw((3.46,0.96)--(8.02,-3.44)--(12.42,1.12)--(7.86,5.52)--cycle);   /* draw figures */ draw((3.46,0.96)--(3.44,-3.36));  draw((3.44,-3.36)--(8.02,-3.44));  draw((8.02,-3.44)--(3.46,0.96));  draw((3.46,0.96)--(-0.86,0.98));  draw((-0.86,0.98)--(-0.88,-3.34));  draw((-0.88,-3.34)--(3.44,-3.36));  draw((3.46,0.96)--(8.02,-3.44));  draw((8.02,-3.44)--(12.42,1.12));  draw((12.42,1.12)--(7.86,5.52));  draw((7.86,5.52)--(3.46,0.96));  draw((5.74,-1.24)--(-0.86,0.98));  draw((5.74,-1.24)--(-0.87,-1.18), linetype("4 4"));  draw((5.74,-1.24)--(7.86,5.52));  draw((5.74,-1.24)--(10.14,3.32), linetype("4 4"));  draw(shift((5.82,-1.21))*xscale(6.99920709795045)*yscale(6.99920709795045)*arc((0,0),1,19.44457562540183,197.63600413408128), linetype("2 2"));   /* dots and labels */ dot((3.46,0.96),dotstyle);  label("$A$", (3.2,1.06), NE * labelscalefactor);  dot((3.44,-3.36),dotstyle);  label("$C$", (3.14,-3.86), NE * labelscalefactor);  dot((8.02,-3.44),dotstyle);  label("$B$", (8.06,-3.8), NE * labelscalefactor);  dot((-0.86,0.98),dotstyle);  label("$Z$", (-1.34,1.12), NE * labelscalefactor);  dot((-0.88,-3.34),dotstyle);  label("$W$", (-1.48,-3.54), NE * labelscalefactor);  dot((12.42,1.12),dotstyle);  label("$X$", (12.5,1.24), NE * labelscalefactor);  dot((7.86,5.52),dotstyle);  label("$Y$", (7.94,5.64), NE * labelscalefactor);  dot((5.74,-1.24),dotstyle);  label("$O$", (5.52,-1.82), NE * labelscalefactor);  clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle);  [/asy]

Solution 2

Let $AC = b$ and $BC = a$ (and we're given that $AB=12$). Draw line segments $YZ$ and $WX$. Now we have cyclic quadrilateral $WXYZ.$

This means that opposite angles sum to $180^{\circ}$. Therefore, $90 + m\angle YZA + 90 - m\angle WXB = 180$. Simplifying carefully, we get $m\angle YZA = m\angle WXB$. Similarly, $m\angle{ZYA}$ = $m\angle{XWB}$.

That means $\triangle ZYA \sim \triangle XWB$.

Setting up proportions, $\dfrac{b}{12}=\dfrac{12}{a+b}.$ Cross-multiplying we get: $b^2+ab=12^2$

But also, by Pythagoras, $b^2+a^2=12^2$, so $ab=a^2 \Rightarrow a=b$

Therefore, $\triangle ABC$ is an isosceles right triangle. $AC=BC=\dfrac{12}{\sqrt{2}}=6\sqrt{2}$, so the perimeter is \[\boxed{\textbf{(C)}\ 12+12\sqrt{2}}\]



Solution 3 - Fakesolve

Temporarily assume that we are as dumb as samrocksnature. Then, we would misread the instructions and draw the squares inside, not outside, the triangle. In addition, we would make a plethora of ridiculous assumptions.

Our diagram would essentially be two squares intersecting in a square, such that a circle can be drawn through the two leftmost vertices of the left square and the two rightmost vertices of the right square. Since the diagonal of the square is given to be $12$, our answer is $12+6\sqrt{2}+6\sqrt{2} \Rightarrow \boxed{C}$.

See Also

2015 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 18
Followed by
Problem 20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions

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