Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME, harder Find the answer

Real numbers a,b,ca, b, c satisfy the equations a+b+c=26,1/a+1/b+1/c=28a+b+c=26,1 / a+1 / b+1 / c=28. Find the value of ab+bc+ca+ac+cb+ba\frac{a}{b}+\frac{b}{c}+\frac{c}{a}+\frac{a}{c}+\frac{c}{b}+\frac{b}{a}

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

Solution

Multiplying the two given equations gives aa+ab+ac+ba+bb+bc+ca+cb+cc=2628=728\frac{a}{a}+\frac{a}{b}+\frac{a}{c}+\frac{b}{a}+\frac{b}{b}+\frac{b}{c}+\frac{c}{a}+\frac{c}{b}+\frac{c}{c}=26 \cdot 28=728 and subtracting 3 from both sides gives the answer, 725.

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