Det [1, 3; 2, 8]

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 = 1 × 8 − 2 × 3.

Careful!

Always down minus up.

Up minus down gives the wrong sign.

2 Down minus up

1 times 8 is 8.

2 times 3 is 6.

8 minus 6 is 2.

So D = 2.

3 Check

Let's check the taking away by adding back: 2 + 6 = 8, the down product.

So D = 2 is right!

Let's look back.

First: multiply the diagonals.

Then: down minus up.

Last: check.