Cn0+Cn1+……+Cnn=2^n
C2n 0+……+C2n,2n=2^2n
所以2^2n-2^n=240
n=4
所以第三项是C42*[x^(1/2)]^(4-2)*[x^(-1/3)]^2=6x^(1/3)
中间项是第5项
=C84*a^4b^4=70a^4b^4