Det [2, 4; 1, 2]

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 × 2 − 1 × 4.

Careful!

Always down minus up.

2 Down minus up

2 times 2 is 4.

1 times 4 is 4.

4 minus 4 is 0.

So D = 0.

3 Check

Let's check the taking away by adding back: 0 + 4 = 4, the down product.

So D = 0 is right!

Let's look back.

First: multiply the diagonals.

Then: down minus up.

Last: check.