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Geometry/Triangle / Incircle

Question

[Geometry -- Triangle / Incircle, Very Hard, TITA]

In triangle ABCABC, AB=14AB = 14, BC=13BC = 13 and CA=15CA = 15. Let MM be the midpoint of side BCBC, and let PP be the point at which the incircle of triangle ABCABC touches side ABAB. The value of 4MP24 \cdot MP^{2} is ____.

Answer

Detailed solution

[Incircle touch point to midpoint of opposite side]

With a=BC=13a = BC = 13, b=CA=15b = CA = 15, c=AB=14c = AB = 14, semi-perimeter s=21s = 21. Tangent length from AA: AP=sa=8AP = s - a = 8.

Median length. By Apollonius for median AMAM from AA to mid-BCBC:

$

AM^{2} = \frac{2b^{2} + 2c^{2} - a^{2}}{4} = \frac{2(225) + 2(196) - 169}{4} = \frac{673}{4}.

$

Angle BAM\angle BAM. Apply the law of cosines in ABM\triangle ABM, with BM=13/2BM = 13/2:

$

BM^{2} = AB^{2} + AM^{2} - 2 AB\cdot AM\cos\angle BAM \Rightarrow \tfrac{169}{4} = 196 + \tfrac{673}{4} - 14\sqrt{673}\cos\angle BAM,

$

$

\Rightarrow 14\sqrt{673}\cos\angle BAM = 322 \Rightarrow \cos\angle BAM = \tfrac{23}{\sqrt{673}}.

$

Apply law of cosines in AMP\triangle AMP: AP=8AP = 8, AM=6732AM = \tfrac{\sqrt{673}}{2}:

$

MP^{2} = 64 + \tfrac{673}{4} - 2 \cdot 8 \cdot \tfrac{\sqrt{673}}{2} \cdot \tfrac{23}{\sqrt{673}} = 64 + \tfrac{673}{4} - 184 = \tfrac{193}{4}.

$

So 4MP2=1934 MP^{2} = 193. Answer: 193.

Why CAT-level: Coordinate geometry works but produces fractions with denominator 169169. The slick path requires combining median formula + law of cosines, an approach not directly signalled by the problem.