To proveLet x and y be non-negative real numbers. lemma: square roots of non-negative real numbers are unique. Proof: For contradiction, suppose for positive real number " a ", p and q are two different non-negative square roots of " a ". Then, => => since , p-q=0, so p=q. So we have reached a contradiction namely, that p and q are NOT different. We show that the squares of and are equal. Therefore and by the lemma, PROVED. Edwin