Let "R" be a commutative ring with identity and "I" and "J" be ideals of "R" such that "I+J=R". Let us prove that "IJ=I\\cap J".
Since "I" is an ideal, "IJ\\subset IR\\subset I". Since "J" is an ideal, "IJ\\subset RJ\\subset J". Therefore, "IJ\\subset I\\cap J".
On the other hand, taking into accounr that "R" is a commutative ring with identity, we conclude that
"I\\cap J\\subset (I\\cap J)R\\subset (I\\cap J)(I+J)= (I\\cap J)I+ (I\\cap J)J\\subset JI+IJ=IJ+IJ=IJ".
Consequently, "IJ=I\\cap J".
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