I need help with part B, please!

Why is the relationship above approximate and not exact?

3) As discussed in class, the coefficients in the virial equation, unlike the a and b coefficients in the van der Waals equation, do not have a simple relationship to the physical properties of a gas. However, for a two term virial equation p=VmRT{1+(B/Vm)+(C/Vm2)} an approximate relationship between the B and C coefficients of eq 3.1 and the a and b coefficients of the van der Waals equation can be found. a) Starting with the van der Waals equation of state p=(Vmb)RTVm2a rewrite equation 3.2 so that it has the same form as eq 3.1. Based on this, find an approximate relationship between the virial equation coefficients B ad C and the van der Walls coefficients a and b. Hint: The following Taylor series may be of use in doing this problem (1x)1=1+x+x2+x3+ b) Why is the relationship found above approximate and not exact? 3) As discussed in class, the coefficients in the virial equation, unlike the a and b coefficients in the van der Waals equation, do not have a simple relationship to the physical properties of a gas. However, for a two term virial equation p=VmRT{1+(B/Vm)+(C/Vm2)} an approximate relationship between the B and C coefficients of eq 3.1 and the a and b coefficients of the van der Waals equation can be found. a) Starting with the van der Waals equation of state p=(Vmb)RTVm2a rewrite equation 3.2 so that it has the same form as eq 3.1. Based on this, find an approximate relationship between the virial equation coefficients B ad C and the van der Walls coefficients a and b. Hint: The following Taylor series may be of use in doing this problem (1x)1=1+x+x2+x3+ b) Why is the relationship found above approximate and not exact