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JAMB Mathematics 2012 Paper

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Question : 20

Total: 48

The binary operation on the set of real numbers is defined by m*n =

\frac{mn}{2}

\in

R. If the identity element is 2, find the inverse of -5

$-\frac{4}{5}$
$-\frac{2}{5}$
4
5

Solution:

Concept:

The inverse of an element is the value that combines with it to give the identity element.

Explanation:

Let the inverse of

-5

be

x

.

Then

(-5) * x = e

, where

e = 2

.

Using the operation

m * n = \frac{mn}{2}

, we have:

\frac{-5 \cdot x}{2} = 2

Multiply both sides by

2

:

-5x = 4

Divide both sides by

-5

:

x = -\frac{4}{5}

So the inverse of

-5

is

-\frac{4}{5}

.

Answer:

Option A:

-\frac{4}{5}

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