Angles in a triangle 45, 45, x

Step by step

The question

Question: find x when the angles in a triangle are 45°, 45° and x.

1 Use the angle fact

The three angles in a triangle always add up to 180°.

So we add them all and make it 180: 45 + 45 + x = 180.

2 Group like terms

The plain numbers go together.

Now we have 90 + x = 180.

3 Remove the 90

We want x by itself.

The 90 is in the way.

To remove 90, we take away 90.

We do it on both sides.

Why both sides?

The = sign means both sides are the same amount.

If we change only one side, they are not the same any more.

90 and − 90 make zero.

Cross them out.

180 minus 90 is 90.

So x = 90.

The angle x is 90°.

4 Check

Let's check: put x = 90 back and add the angles.

They add up to 180°.

So x = 90 is right!

Let's look back.

First: use the angle fact.

Then: group like terms.

Then: remove the 90.

Last: check.