第3项为C(n,3-1)*x^(n+1-3)*[(2/x^2)^(3-1)]
=C(n,3-1)*x^(n-2)*[(2/x^2)^2]
=4C(n,3-1)*x^[(n-2)-2*2]
=4C(n,3-1)*x^(n-6)
为常数
n-6=0 n=6
展开共6+1=7项中间项为第4项
中间项=C(6,4)=C(6,4-1)*x^(6+1-4)*[(2/x^2)^(4-1)]
=C(6,3)*(x^3)*[(2/x^2)^3]
=160/(x^3)
第3项为C(n,3-1)*x^(n+1-3)*[(2/x^2)^(3-1)]
=C(n,3-1)*x^(n-2)*[(2/x^2)^2]
=4C(n,3-1)*x^[(n-2)-2*2]
=4C(n,3-1)*x^(n-6)
为常数
n-6=0 n=6
展开共6+1=7项中间项为第4项
中间项=C(6,4)=C(6,4-1)*x^(6+1-4)*[(2/x^2)^(4-1)]
=C(6,3)*(x^3)*[(2/x^2)^3]
=160/(x^3)