Det [2, 1; 1, 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 = 2 × 3 − 1 × 1.

Careful!

Always down minus up.

Up minus down gives the wrong sign.

2 Down minus up

2 times 3 is 6.

1 times 1 is 1.

6 minus 1 is 5.

So D = 5.

3 Check

Let's check the taking away by adding back: 5 + 1 = 6, the down product.

So D = 5 is right!

Let's look back.

First: multiply the diagonals.

Then: down minus up.

Last: check.