3.设(X1,X2,X3,X4,X 5)为正态总体N (0,1)的样本,试确定c,使随机变量 Y=-|||-dfrac (c({X)_(1)+(X)_(2))}(sqrt {{{X)_(3)}^2+({X)_(4)}^2+({X)_(5)}^2}} 服从t分布.