Exterior angle 150, inside angles 90 and x

Step by step

The question

Question: if the exterior angle is 150° and the far inside angles are 90° and x°, what is x?

The exterior angle is outside the triangle, on the pushed-out side.

1 Use the rule

The exterior angle is the two far inside angles added together.

So 90 + x = 150.

Careful!

The outside angle is not inside the triangle.

It does not go in the 180° sum.

2 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.

150 − 90 = 60.

So x = 60!

3 Check

Let's check: with x = 60, the inside angles are 90° and 60°.

Together 150°, the outside angle.

So the answer is right!

Let's look back.

First: use the rule.

Then: remove the 90.

Last: check.