Simplify -2i × 5i

Step by step

The question

Question: work out −2i × 5i.

No real number times itself is negative.

So we use a new number: i, the square root of minus 1.

So i squared is minus 1.

1 Multiply each part

−2i times 5i is minus 10i².

2 Change i² to −1

i squared is minus 1.

So minus 10i² becomes 10.

Minus times minus is plus.

Now we have 10.

So the answer is 10.

3 Check

Let's check: multiply the other way round, 5i × −2i.

Each part times each part again, and i² is −1.

We get 10 again.

So the answer is right!

Let's look back.

First: multiply each part.

Then: change i² to −1.

Last: check.