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Algebra Difficulty 4.9 AIME Prove it Estonia

Let aa, bb and cc be real numbers for which abc=1abc = 1. Prove that
11+a2014+11+b2014+11+c2014>1. \frac{1}{1+a^{2014}} + \frac{1}{1+b^{2014}} + \frac{1}{1+c^{2014}} > 1.

Solutions — 3

Solution 1

Let a2014=ua^{2014} = u, b2014=vb^{2014} = v and c2014=wc^{2014} = w; then abc=1abc = 1 gives that uvw=1uvw = 1. As the numerators of the l.h.s. of the inequality to be proven are positive, the inequality is equivalent to
(1+v)(1+w)+(1+w)(1+u)+(1+u)(1+v)>(1+u)(1+v)(1+w). (1+v)(1+w) + (1+w)(1+u) + (1+u)(1+v) > (1+u)(1+v)(1+w).

By expanding, simplifying and using uvw=1uvw = 1, we get 1+u+v+w>01 + u + v + w > 0. But this is true, since uu, vv and ww are positive.

Solution 2

Let a2014=ua^{2014} = u, b2014=vb^{2014} = v and c2014=wc^{2014} = w. Then abc=1abc = 1 gives that uvw=1uvw = 1. Therefore there exist positive real numbers xx, yy, zz such that u=xyu = \frac{x}{y}, v=yzv = \frac{y}{z} and w=zxw = \frac{z}{x}.
The inequality can then be written as 11+xy+11+yz+11+zx>1\frac{1}{1+\frac{x}{y}} + \frac{1}{1+\frac{y}{z}} + \frac{1}{1+\frac{z}{x}} > 1 which is equivalent to
yx+y+zy+z+xz+x>1. \frac{y}{x+y} + \frac{z}{y+z} + \frac{x}{z+x} > 1.
But this inequality can be obtained by adding the obvious inequalities yx+y>yx+y+z\frac{y}{x+y} > \frac{y}{x+y+z}, zy+z>zx+y+z\frac{z}{y+z} > \frac{z}{x+y+z} and xz+x>xx+y+z\frac{x}{z+x} > \frac{x}{x+y+z}.

Solution 3

Let a2014=ua^{2014} = u, b2014=vb^{2014} = v and c2014=wc^{2014} = w; then abc=1abc = 1 gives that uvw=1uvw = 1. W.l.o.g., let ww be the greatest of uu, vv, ww. Then w1w \ge 1, because otherwise uu, vv, ww would all be less than 11 and their product could not be 11. Thus uv1uv \le 1. Now
11+u+11+v1=(1+v)+(1+u)(1+u)(1+v)(1+u)(1+v)=1uv(1+u)(1+v)0. \frac{1}{1+u} + \frac{1}{1+v} - 1 = \frac{(1+v) + (1+u) - (1+u)(1+v)}{(1+u)(1+v)} = \frac{1-uv}{(1+u)(1+v)} \ge 0.
Thus 11+u+11+v1\frac{1}{1+u} + \frac{1}{1+v} \ge 1, from which the desired inequality can be concluded.

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