Question 21

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Jambmaths question: 

Question 21
A binary operation $\Delta $is defined by $a\Delta b=a+b+1$for any real number a and b, Find the inverse of the real number 7 under the operation$\Delta $, if the identity element is -1

Option A: 

9

Option B: 

5

Option C: 

–7

Option D: 

–9

Jamb Maths Solution: 

$\begin{align}  & \text{Let the inverse be }{{a}^{-1}} \\ & a\Delta {{a}^{-1}}=e\text{    (}e\text{ is the identity element)} \\ & \text{7}\Delta {{a}^{-1}}=-1 \\ & =7+{{a}^{-1}}+1=-1 \\ & {{a}^{-1}}=-1-1-7 \\ & {{a}^{-1}}=-9 \\\end{align}$

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