Question about thermal expansion coefficient
 

Hi! guys!
I got a question about thermal expansion coefficient.

for ideal gas, the thermal expansion coefficient equals to 1/T, what does this T mean? absolute temperature?

So, if Tw=300K, Th=1100K, how to calculate the thermal expansion coefficient?
T=0.5*(300+1100)? or just simply choose 273.15 as the denominator?

Can anyone help me? thanks!