Let * be binary operation defined on R by \(\rm p * q=\frac{p+q}{2}, \forall \) p, q ∈ R. The operation is:

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  1. commutative but not associative
  2. associative but not commutative
  3. commutative and associative
  4. neither associative nor commutative

Answer (Detailed Solution Below)

Option 1 : commutative but not associative
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Given:

Let * be binary operation defined on R by \(\rm p * q=\frac{p+q}{2}, \forall \) p, q ∈ R

Concept:

If \(\rm p*q = q*p\) then relation is commutative.

If \(\rm (p*q)*r = p*(q*r)\) then relation is associative .

Calculation:

Let * be binary operation defined on R by \(\rm p * q=\frac{p+q}{2}, \forall \) p, q ∈ R

Commutative:

Let p, q ∈ R

Now,

\(\rm p*q=\frac{p+q}{2}=\frac{q+p}{2}=q*p\)

Hence R is commutative.

Associative :

Let p, q, r ∈ R then

\(\rm (p*q)*r=\frac{p+q}{2}*r=\frac{p+q+2r}{4}\)

\(\rm p*(q*r)=p*\frac{q+r}{2}=\frac{2p+q+r}{4}\)

then \(\rm (p*q)*r\ne p*(q*r)\)

Hence R is not associative.

Hence the option (1) correct .

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