有a.maths 唔識.....

using the general term nCr*(a)^(n-r)*(b)^r
in this case a=2x , b=-1/x,n=10
notice that higher the n-r is, higher is the power of x in the term is.
as the expansion in arrange in descending powers of x, take r=2

the ans:70*(2x)^7*(-1/x)^3=-11520x^6

In general n-1 term will be (2x)^(10-n) * (-1/x)^(n) * 10Cn

Hence the 3rd term = (2x)^8 * (-1/x)^2* 10C2 = ( 256x^6)*45= 11520x^6

用general term nCr(a)^(n-r)(b)^r where n=10