Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Find the answer

G9.1 The lengths of the 3 sides of LMN\triangle L M N are 8,15 and 17 respectively.
If the area of LMN\triangle L M N is AA, find AA.

G9.2 If rr is the length of the radius of the circle inscribed in LMN\triangle L M N, find rr.

A number or a short expression. Spacing and $ signs are ignored.

Solution

82+152=64+225=289=172 8^{2}+15^{2}=64+225=289=17^{2}
LMN\therefore \triangle L M N is a right-angled triangle
A=8×152=60 A=\frac{8 \times 15}{2}=60

Reference: 1989 HG9
Let OO be the centre and the radius of the circle be rr, which touches the triangle at C,DC, D and EE.
OCLM,ODMN,OELNO C \perp L M, O D \perp M N, O E \perp L N (tangent \perp radius)
ODMCO D M C is a rectangle (which consists of 3 right angles)
OC=r=OD (radii)  O C=r=O D \text { (radii) }
OCMD\Rightarrow O C M D is a square.
CM=MD=r (opp. sides, rectangle) LC=15r,ND=8rLE=LC=15r,NE=ND=8r (tangent from ext. pt.) LE+NE=LN15r+8r=17r=3 \begin{array}{l} C M=M D=r \text { (opp. sides, rectangle) } \\ L C=15-r, N D=8-r \\ L E=L C=15-r, N E=N D=8-r \text { (tangent from ext. pt.) } \\ L E+N E=L N \\ \Rightarrow 15-r+8-r=17 \\ \Rightarrow r=3 \end{array}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.