MCQ
Choose the correct answer out of the given four options.Let $*$ be binary operation defined on $R$ by $a * b = 1 + ab \forall a, b \in R.$ Then the operation $*$ is:
  • Commutative but not associative.
  • B
    Associative but not commutative.
  • C
    Neither commutative nor associative.
  • D
    Both commutative and associative.

Answer

Correct option: A.
Commutative but not associative.
We are given that, $a^ * b = 1 + ab \forall a, b \in R$
Consider$, a^ * b = ab + 1$
$= ba + 1$
$= b^ * a$
Hence$, *$ is a communicative binary operation.
Also$, a^ * (b^ * c) = a^ * (bc + 1) [\because b^ * c = bc + 1]$
$= a(bc + 1) + 1$
$= a + abc + 1$
Now$, (a^ * b)^ * c = (ab + 1)^ * c [\because a^ * b = ab + 1]$
$= (1 + ab)c + 1$
$= c + abc +1$
Now, $\text{a}+\text{abc}+1\neq\text{c}+\text{abc}+1$
$\Rightarrow\ \text{a}\ ^*\ (\text{b}\ ^* \ \text{c})\neq(\text{a}\ ^* \ \text{b})\ ^* \ \text{c}$
Therefore$, *$ is not associative.
Hence$, *$ is communicative but not associative.

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