Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it Bulgaria

Problem:
Find the maximum of the function
f(x)=lgxlgx2+lgx3+3lg2x+lgx2+2 f(x) = \frac{\lg x \cdot \lg x^{2} + \lg x^{3} + 3}{\lg^{2} x + \lg x^{2} + 2}
and the values of xx, when it is attained.

Solution

Solution:
The domain of f(x)f(x) is x>0x > 0. Setting y=lgxy = \lg x gives
F(y)=2y2+3y+3y2+2y+2 F(y) = \frac{2y^{2} + 3y + 3}{y^{2} + 2y + 2}
Since the denominator is positive, the function F(y)F(y) is defined for all real yy.
Let MM be the desired value of f(x)f(x) (if it exists). Then for any real yy we have
2y2+3y+3y2+2y+2M2y2+3y+3My2+2My+2M(2M)y2+(32M)y+(32M)0 \begin{gathered} \frac{2y^{2} + 3y + 3}{y^{2} + 2y + 2} \leq M \\ 2y^{2} + 3y + 3 \leq M y^{2} + 2M y + 2M \\ (2 - M) y^{2} + (3 - 2M) y + (3 - 2M) \leq 0 \end{gathered}
Therefore 2M<02 - M < 0 and D=(32M)(32M8+4M)=(32M)(2M5)0D = (3 - 2M)(3 - 2M - 8 + 4M) = (3 - 2M)(2M - 5) \leq 0. Hence M2.5M \geq 2.5.
Note that for M=2.5M = 2.5 the above inequality becomes 0.5y22y20-0.5 y^{2} - 2y - 2 \leq 0, i.e. y2+4y+4=(y+2)20y^{2} + 4y + 4 = (y + 2)^{2} \geq 0 and the equality is attained only if y=2y = -2, i.e. for x=0.01x = 0.01.
Therefore the maximum of the function equals 2.52.5 and it is attained for x=0.01x = 0.01 only.

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