Triangles
Pythagoras
Right angled triangles appear all over the place in physics:

If you know the length of two sides then you can find the length of the third
Pythagoras: Example A
This example shows how you can find the length of the third side of a triangle.
Trigonometry (Sin, Cos and Tan)
If you have a right angled triangle then you can do clever things:

If you know the length of one side and an angle you can calculate another side

If you know two sides you can calculate an angle

Once you know one angle, or two sides, you can calculate the other angle and other side
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