lesson
Updated 6 days ago · 2 views
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 (∘).
A triangle's angles always add to 180∘, and because any four-sided shape cuts into two triangles, its angles add to 180∘+180∘=360∘.
📊Interactive diagram
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∘ to find the missing piece.
📊Interactive diagram
Let's check our steps: 100∘+80∘+90∘=270∘, and 360∘−270∘=90∘, so the missing corner x is 90∘.
A common trap is subtracting from 180∘ by mistake — always remember that a four-sided shape has two triangles inside, so its target sum is always 360∘!