Use the Pythagorean Theorem to answer this. Folding the paper along its diagonal produces two triangles; each one has shorter side 8.5 in and longer side 11 in. According to the Pyth. Thm., (8.5)^2 + 11^2 = 193.25, which results in the diagonal length √193.25, or approx. 13.9 in.
