在目标c中添加两个double值的问题

I'm programming an app for iphone and ipad and my program requires adding two double values to get a single double value. The problems is when one of the double values is fairly large (eg: 2^100) and the other one is very small like 1 or 2, the result of adding those two double values is wrong or it doesn't even do the addition. Does anyone know why that is and if there's a way around it. Thank you.I'm programming an app for iphone and ipad and