Step by step
The question
Question: find x.
ABCD is a cyclic quadrilateral.
The opposite angles at A and C are 100° and x.
1 Use the circle rule
All four corners, A, B, C and D, are on the circle.
This is called a cyclic quadrilateral.
Join B and D to the center, O.
The angle at A stands on the arc through C.
The circle rule: the angle at the center on that arc is double: 2 times A.
The angle at C stands on the other arc.
Its angle at the center is 2 times C.
Together, the two angles at the center go all the way round: 360°.
Half of that: A and C add up to 180°.
Opposite angles always add up to 180°.
So we add them and make it 180: 100 + x = 180.
Careful!
Opposite angles are equal in a parallelogram.
In a cyclic quadrilateral they are not equal.
They add up to 180°.
2 Remove the 100
We want x by itself.
The 100 is in the way.
To remove 100, we take away 100.
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.
100 and − 100 make zero.
Cross them out.
180 − 100 = 80.
So x = 80!
3 Check
Let's check: put x = 80 back and add the opposite angles.
They add up to 180°.
So x = 80 is right!
Let's look back.
First: use the circle rule.
Then: remove the 100.
Last: check.