1964 AHSME Problems/Problem 39

Problem

The magnitudes of the sides of triangle $ABC$ are $a$, $b$, and $c$, as shown, with $c\le b\le a$. Through interior point $P$ and the vertices $A$, $B$, $C$, lines are drawn meeting the opposite sides in $A'$, $B'$, $C'$, respectively. Let $s=AA'+BB'+CC'$. Then, for all positions of point $P$, $s$ is less than:

$\textbf{(A) }2a+b\qquad\textbf{(B) }2a+c\qquad\textbf{(C) }2b+c\qquad\textbf{(D) }a+2b\qquad \textbf{(E) }$ $a+b+c$


[asy] import math;  pair A = (0,0), B = (1,3), C = (5,0), P = (1.5,1);  pair X = extension(B,C,A,P), Y = extension(A,C,B,P), Z = extension(A,B,C,P);  draw(A--B--C--cycle); draw(A--X); draw(B--Y); draw(C--Z); dot(P); dot(A); dot(B); dot(C); label("$A$",A,dir(210)); label("$B$",B,dir(90)); label("$C$",C,dir(-30)); label("$A'$",X,dir(-100)); label("$B'$",Y,dir(65)); label("$C'$",Z,dir(20)); label("$P$",P,dir(70)); label("$a$",X,dir(80)); label("$b$",Y,dir(-90)); label("$c$",Z,dir(110)); [/asy]

Solution

We know that in a $\triangle DEF$, if $\angle D \le \angle E$ then $EF \le DF$, we can use this fact in the different triangles to form inequalities, and then add the inequalities.

In $\triangle ABC$, since $c \le b \le a$, we have $\angle C \le \angle B \le \angle A$ by the above argument.


Now, $\angle AXC > \angle B \ge \angle C$, hence we have $AC > AX \implies b > AX$

And, $\angle BYC > \angle A \ge \angle C$, hence we have $BY > BC \implies a > BY$

And, $\angle CZB > \angle A \ge \angle B$, hence we have $BC > ZC \implies a > ZC$

Finally, adding all three inequalities, we have $b + a + a > AX + BY + ZC \implies AX + BY + CZ < \textbf{2a + b}$

See Also

1964 AHSC (ProblemsAnswer KeyResources)
Preceded by
Problem 38
Followed by
Problem 40
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