Problem: Find the maximum of the function f(x)=lg2x+lgx2+2lgx⋅lgx2+lgx3+3 and the values of x, when it is attained.
Solution
Solution: The domain of f(x) is x>0. Setting y=lgx gives F(y)=y2+2y+22y2+3y+3 Since the denominator is positive, the function F(y) is defined for all real y. Let M be the desired value of f(x) (if it exists). Then for any real y we have y2+2y+22y2+3y+3≤M2y2+3y+3≤My2+2My+2M(2−M)y2+(3−2M)y+(3−2M)≤0 Therefore 2−M<0 and D=(3−2M)(3−2M−8+4M)=(3−2M)(2M−5)≤0. Hence M≥2.5. Note that for M=2.5 the above inequality becomes −0.5y2−2y−2≤0, i.e. y2+4y+4=(y+2)2≥0 and the equality is attained only if y=−2, i.e. for x=0.01. Therefore the maximum of the function equals 2.5 and it is attained for x=0.01 only.
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Source: MathNet,
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