Det [9, 4; 5, 3]

Step by step

The question

Question: find the determinant of this matrix.

The determinant is one number we get from a square matrix.

We write the matrix between two straight lines, and call it D.

1 Multiply the diagonals

So D = 9 × 3 − 5 × 4.

Careful!

Always down minus up.

Up minus down gives the wrong sign.

2 Down minus up

9 times 3 is 27.

5 times 4 is 20.

27 minus 20 is 7.

So D = 7.

3 Check

Let's check the taking away by adding back: 7 + 20 = 27, the down product.

So D = 7 is right!

Let's look back.

First: multiply the diagonals.

Then: down minus up.

Last: check.