Maths Olympiad Prep

Library / /206 of 520

Geometry Difficulty 6.8 National olympiad Find the answer

In triangle ABC\triangle ABC, the points A,B,CA', B', C' are on sides BC,AC,ABBC, AC, AB respectively. Also, AA,BB,CCAA', BB', CC' intersect at the point OO(they are concurrent at OO). Also, AOOA+BOOB+COOC=92\frac {AO}{OA'}+\frac {BO}{OB'}+\frac {CO}{OC'} = 92. Find the value of AOOA×BOOB×COOC\frac {AO}{OA'}\times \frac {BO}{OB'}\times \frac {CO}{OC'}.

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

Solution

1. Define Ratios:
Let
ACBA=α,BACB=β,CBAC=γ. \frac{A'C}{BA'} = \alpha, \quad \frac{B'A}{CB'} = \beta, \quad \frac{C'B}{AC'} = \gamma.
According to Ceva's Theorem, for the cevians AAAA', BBBB', and CCCC' to be concurrent, the product of these ratios must be equal to 1:
αβγ=1. \alpha \beta \gamma = 1.

2. Apply Van Aubel's Theorem:
Van Aubel's Theorem states that:
AOOA=β+1γ,BOOB=γ+1α,COOC=α+1β. \frac{AO}{OA'} = \beta + \frac{1}{\gamma}, \quad \frac{BO}{OB'} = \gamma + \frac{1}{\alpha}, \quad \frac{CO}{OC'} = \alpha + \frac{1}{\beta}.

3. Sum of Ratios:
Given in the problem:
AOOA+BOOB+COOC=92. \frac{AO}{OA'} + \frac{BO}{OB'} + \frac{CO}{OC'} = 92.

4. Product of Ratios:
We need to find the value of:
AOOA×BOOB×COOC. \frac{AO}{OA'} \times \frac{BO}{OB'} \times \frac{CO}{OC'}.
Using the expressions from Van Aubel's Theorem, we have:
AOOA×BOOB×COOC=(β+1γ)(γ+1α)(α+1β). \frac{AO}{OA'} \times \frac{BO}{OB'} \times \frac{CO}{OC'} = \left(\beta + \frac{1}{\gamma}\right) \left(\gamma + \frac{1}{\alpha}\right) \left(\alpha + \frac{1}{\beta}\right).

5. Expand the Product:
Expanding the product, we get:
(β+1γ)(γ+1α)(α+1β). \left(\beta + \frac{1}{\gamma}\right) \left(\gamma + \frac{1}{\alpha}\right) \left(\alpha + \frac{1}{\beta}\right).
Using the identity αβγ=1\alpha \beta \gamma = 1, we can simplify:
(β+1γ)(γ+1α)(α+1β)=βγα+βγ1β+β1αα+1γγα+1γγ1α+1γ1βα+1γ1β1α. \left(\beta + \frac{1}{\gamma}\right) \left(\gamma + \frac{1}{\alpha}\right) \left(\alpha + \frac{1}{\beta}\right) = \beta \gamma \alpha + \beta \gamma \frac{1}{\beta} + \beta \frac{1}{\alpha} \alpha + \frac{1}{\gamma} \gamma \alpha + \frac{1}{\gamma} \gamma \frac{1}{\alpha} + \frac{1}{\gamma} \frac{1}{\beta} \alpha + \frac{1}{\gamma} \frac{1}{\beta} \frac{1}{\alpha}.
Simplifying further:
=αβγ+γ+β+α+1α+1β+1γ+1αβγ. = \alpha \beta \gamma + \gamma + \beta + \alpha + \frac{1}{\alpha} + \frac{1}{\beta} + \frac{1}{\gamma} + \frac{1}{\alpha \beta \gamma}.
Since αβγ=1\alpha \beta \gamma = 1, we have:
=1+γ+β+α+1α+1β+1γ+1. = 1 + \gamma + \beta + \alpha + \frac{1}{\alpha} + \frac{1}{\beta} + \frac{1}{\gamma} + 1.
Combining terms:
=2+(β+1γ)+(γ+1α)+(α+1β). = 2 + \left(\beta + \frac{1}{\gamma}\right) + \left(\gamma + \frac{1}{\alpha}\right) + \left(\alpha + \frac{1}{\beta}\right).
Given that:
AOOA+BOOB+COOC=92, \frac{AO}{OA'} + \frac{BO}{OB'} + \frac{CO}{OC'} = 92,
we substitute:
=2+92=94. = 2 + 92 = 94.

The final answer is 94\boxed{94}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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