Step by step
The question
Question: find x.
ABCD is a cyclic quadrilateral.
The opposite angles at A and C are x and 72°.
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: x + 72 = 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 + 72
We want x by itself.
The + 72 is in the way.
To remove + 72, we take away 72.
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.
+ 72 and − 72 make zero.
Cross them out.
180 − 72 = 108.
So x = 108!
3 Check
Let's check: put x = 108 back and add the opposite angles.
They add up to 180°.
So x = 108 is right!
Let's look back.
First: use the circle rule.
Then: remove the + 72.
Last: check.