using the identity (a+b)(a-b)=a^2-b^2

mutlitply and divide the given question by (2-1).. {(2-1)(2+1)(2^2+1)(2^4+1)(2^8+1)}/(2-1)= (2^2-1)(2^2+1)(2^4+1)(2^8+1)= (2^4-1)(2^4+1)(2^8+1)= (2^8-1)(2^8+1)= 2^16-1