Difference between revisions of "1987 AIME Problems/Problem 9"

(Solution)
(Video Solution by MegaMath)
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label("$P$", P, NE);
 
label("$P$", P, NE);
 
</asy>
 
</asy>
 
==Video Solution by MegaMath==
 
https://www.youtube.com/watch?v=r9HFvwcgkvw
 
  
 
== Solution ==
 
== Solution ==

Revision as of 17:44, 5 February 2024

Problem

Triangle $ABC$ has right angle at $B$, and contains a point $P$ for which $PA = 10$, $PB = 6$, and $\angle APB = \angle BPC = \angle CPA$. Find $PC$.

[asy] unitsize(0.2 cm);  pair A, B, C, P;  A = (0,14); B = (0,0); C = (21*sqrt(3),0); P = intersectionpoint(arc(B,6,0,180),arc(C,33,0,180));  draw(A--B--C--cycle); draw(A--P); draw(B--P); draw(C--P);  label("$A$", A, NW); label("$B$", B, SW); label("$C$", C, SE); label("$P$", P, NE); [/asy]

Solution

Let $PC = x$. Since $\angle APB = \angle BPC = \angle CPA$, each of them is equal to $120^\circ$. By the Law of Cosines applied to triangles $\triangle APB$, $\triangle BPC$ and $\triangle CPA$ at their respective angles $P$, remembering that $\cos 120^\circ = -\frac12$, we have

\[AB^2 = 36 + 100 + 60 = 196, BC^2 = 36 + x^2 + 6x, CA^2 = 100 + x^2 + 10x\]

Then by the Pythagorean Theorem, $AB^2 + BC^2 = CA^2$, so

\[x^2 + 10x + 100 = x^2 + 6x + 36 + 196\]

and

\[4x = 132 \Longrightarrow x = \boxed{033}.\]

Note

This is the Fermat point of the triangle.

See also

1987 AIME (ProblemsAnswer KeyResources)
Preceded by
Problem 8
Followed by
Problem 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
All AIME Problems and Solutions

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