1.
In the expansion of (21/x+31/x)15\left(\sqrt{2}^{1/x} + \sqrt{3}^{1/x}\right)^{15}, if the ratio of the 15th term from the beginning to the 15th term from the end is 1:(15?)1:\binom{15}{?} (as given), then the value of 15C3^{15}C_{3} is: