Mathematics - Sequence & Series Question with Solution | TestHub

With $\mathrm{a}+\mathrm{d}=\mathrm{b}+\mathrm{c}$ and $\mathrm{b}\ne \mathrm{c},1/\mathrm{b}+1/\mathrm{c}>4/(\mathrm{b}+\mathrm{c})=4/(\mathrm{a}+\mathrm{d})$. The convexity of 1/t for positive t shows the more spread-out outer pair a,d has $1/\mathrm{a}+1/\mathrm{d}>1/\mathrm{b}+1/\mathrm{c}$.