109. Find all real numbers x,y,z,w such that x+y+z=23,4x−1+4y−1+4z−1⩾2+3w−2 (2007-2008 Hungarian Mathematical Olympiad)
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
109. By the mean inequality, 34x−1+4y−1+4z−1⩽34x−1+4y−1+4z−1=1
Equality holds if and only if x=y=z=21. Therefore, 2+3w−2⩽4x−1+4y−1+4z−1⩽3, hence, w−2⩽0. Thus, w=2, and all the equalities hold. At this point, x=y=z=21,w=2.
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