Let F be a filed and S be a subring of F,To prove that S is an integral domain we must show that S is 1- S is commutative ring.2- S not have a zero divisor.Since F is commutative ring then also the subring S is commutativeLet S have a zero divisor, then there exists , and such that ab=0 since F is filed and there exists with , ab=0abb-1=0b-1a(1)=0a=0which is contradiction with Therefore S not have a zero divisor. And it is an integral domain.