AI Tutor
Imagine folding a square piece of paper right down the middle from corner to corner. You suddenly get two matching triangles!
Every four-sided flat shape is called a quadrilateral, and all four of its inside corners add up to one special number.

The 360-Degree Secret

An angle is the amount of turn between two lines that meet at a corner, measured in degrees (∘^\circ).
A triangle's angles always add to 180∘180^\circ, and because any four-sided shape cuts into two triangles, its angles add to 180∘+180∘=360∘180^\circ + 180^\circ = 360^\circ.
What if you know three corners of a four-sided shape, but one corner is hiding?

Finding the Missing Angle

To find a missing angle, first add together the three angles you already know.
Then, subtract that total from 360∘360^\circ to find the missing piece.
Let's check our steps: 100∘+80∘+90∘=270∘100^\circ + 80^\circ + 90^\circ = 270^\circ, and 360∘−270∘=90∘360^\circ - 270^\circ = 90^\circ, so the missing corner xx is 90∘90^\circ.
A common trap is subtracting from 180∘180^\circ by mistake — always remember that a four-sided shape has two triangles inside, so its target sum is always 360∘360^\circ!