AlgebraDifficulty 7.5National olympiad, round 2Prove itBaltic Way
Let a,b,c,d be positive numbers such that abcd=1. Prove the inequality a+2b+3c+101+b+2c+3d+101+c+2d+3a+101+d+2a+3b+101≤1.
Solution
Let x,y,z,t be positive numbers such that a=x4,b=y4,c=z4,d=t4. By AM-GM inequality x4+y4+z4+1≥4xyz, y4+z4+1+1≥4yz and z4+1+1+1≥4z. Therefore we have the following estimation for the first fraction x4+2y4+3z4+101≤4xyz+4yz+4z+41=2xyz+yz+z+11. Transform analogous estimations for the other fractions: b+2c+3d+101c+2d+3a+101d+2a+3b+101≤2yzt+zt+t+11=2tyz+z+1+xyz1=2xyz+yz+z+1xyz;≤2ztx+tx+x+11=2txz+1+xyz+yz1=2xyz+yz+z+1yz;≤2txy+xy+y+11=2txy1+xyz+yz+z1=2xyz+yz+z+1z. Thus, the sum does not exceed 2xyz+yz+z+11+xyz+yz+z. It remains to apply inequality α+β+γ+δ≤2α+β+γ+δ, which can be easily proven by taking squares or derived from inequality between arithmetical and quadratic means.
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Source: MathNet,
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