（X2+1/X）^n=C(n,0)x^2n+C(n,1)x^(2n-2)）b^1+…+C（n,r）x^(2n-2r)(1/x)^r+…+C(n,n)(1/x)^n
常数项是C(n,r)
(2n-2r)=r,
n=3/2r,
n=6,r=4,
C(6,4)=6*5/2=15