Re: HSC 2015 4U Marathon
Sorry for grammar and i have tried my best to condense working
i) (x-y)^2 > 0 \iff x^2 + y^2 \geq 2xy, \quad x,y >0 \tag{1}
ii) From (1)
x^2 -> x, y^2 -> y, z^2 -> z and divide give
\frac{1}{x+y} \leq 2 \sqrt{xy} \tag{2}
Use (2) and change (x,y) -> (a,b), (a,c) and...