  In this paper, we find some new sharp bounds for $\left(\sin x\right) /x$,
which unify and refine Jordan, Adamovi\'{c}-Mitrinovi\'{c}and and Cusa's
inequalities. As applications of main results, some new Shafer-Fink type
inequalities for arc sine function and ones for certain bivariate means are
established, and a simpler but more accurate estimate for sine integral is
derived.
