Vietnam National Olympiad Problem 1

Algebra Level 5

Determine the number of solutions of the simultaneous equations x 2 + y 3 = 29 x^2 + y^3 = 29 and log 3 x log 2 y = 1 \log_3 x \cdot \log_2 y = 1 .


The answer is 2.

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2 solutions

Chew-Seong Cheong
Mar 21, 2015

Since x 2 + y 3 = 29 y = 29 x 2 3 x^2+y^3=29\quad \Rightarrow y = \sqrt [3] {29-x^2} .

log 3 x log 2 y = 1 log x log y log 3 log 2 = 1 log x log 29 x 2 3 = log 3 log 2 \Rightarrow \log_3 {x} \log_2 {y} = 1 \quad \Rightarrow \dfrac {\log {x} \log {y}}{ \log {3} \log {2}} = 1 \\ \Rightarrow \log {x} \log {\sqrt [3] {29-x^2}} = \log {3} \log {2}

Plotting f ( x ) = log x log 29 x 2 3 log 3 log 2 f(x) = \log {x} \log {\sqrt [3] {29-x^2}} - \log {3} \log {2} , we get the following graph and see that there are 2 \boxed{2} roots x 2 x \approx 2 and x 5 x \approx 5 .

Any other method

Ram Sita - 3 years, 8 months ago
Kanishk Arora
Apr 8, 2015

excellent problem

I do believe this is not a solution.

A Former Brilliant Member - 6 years, 2 months ago

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