X1+X2+X3~N(0,3) √3/3(X1+X2+X3)~N(0,1)
X4+X5+X6~N(0,3) √3/3(X4+X5+X6)~N(0,1)
√3/3(X1+X2+X3)的平方+√3/3(X4+X5+X6)的平方~X(2)
c=√3/3