Let a, b, c be positive real numbers such that a+b+c=1. Prove that 1+a7+2b+1+b7+2c+1+c7+2a≥469. When does equality hold?
Solution
Solution:
The inequality can be written as: 1+a5+2(1+b)+1+b5+2(1+c)+1+c5+2(1+a)≥469. We substitute 1+a=x, 1+b=y, 1+c=z. So, we have to prove the inequality x5+2y+y5+2z+z5+2x≥469⇔5(x1+y1+z1)+2(xy+yz+zx)≥469 where x,y,z>1 real numbers and x+y+z=4. We have - 3x+y+z≥x1+y1+z13⇔x1+y1+z1≥x+y+z9⇔x1+y1+z1≥49 - xy+yz+zx≥3⋅3xy⋅yz⋅zx=3
Thus, x5+2y+y5+2z+z5+2x=5(x1+y1+z1)+2(xy+yz+zx)≥5⋅49+2⋅3=469. The equality holds when (x=y=z,xy=yz=zx,x+y+z=4), thus x=y=z=34, i.e. a=b=c=31.
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