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A binary operation * is defined on the set of real numbers, ...

A binary operation * is defined on the set of real numbers, by ab=ab+ba. If (x+1)(x1)=4, find the value of x. 

A.

6

B.

5

C.

4

D.

3

Correct answer is D

(x+1)(x1)=4x+1x1+x1x+1=4

(x+1)(x+1)+(x1)(x1)(x1)(x+1)

= x+2x+1+x2x+1x12x+2x1=4

2x+2=4x4

x = 3