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Problem 141: (2021 AMC 10b, 21)
A square piece of paper has side length $1$ and vertices $A,B,C,$ and $D$ in that order. As shown in the figure, the paper is folded so that vertex $C$ meets edge $\overline{AD}$ at point $C'$, and edge $\overline{AB}$ at point $E$. Suppose that $C'D = \frac{1}{3}$. What is the perimeter of triangle $\bigtriangleup AEC' ?$ $\textbf{(A)} ~2 \qquad$ $\textbf{(B)} ~1+\frac{2}{3}\sqrt{3} \qquad$ $\textbf{(C)} ~\frac{13}{6} \qquad$ $\textbf{(D)} ~1 + \frac{3}{4}\sqrt{3} \qquad$ $\textbf{(E)} ~\frac{7}{3}$
Solution
Exercise 141: Solution
A square piece of paper has side length $1$ and vertices $A,B,C,D$ in order. The paper is folded so that vertex $C$ meets edge $\overline{AD}$ at $C'$, edge $\overline{BC}$ meets edge $\overline{AB}$ at $E$, and the fold crease meets $\overline{DC}$ at $F$. Suppose $C'D=\frac15$. What is the perimeter of $\triangle DFC'$?
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