These are the Theorems you must be familiar with. You DO NOT need to know how to prove these theorems, however ...

You must ...

• Recognise them
• Understand them
• Know how to use them in your geometry problems.

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## Theorems you MUST know and Understand

### Vertically Opposite Angles are Equal in Measure

1

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-1.mp4

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### In an Isoceles Triangle, Angles Opposite Equal Sides are Equal in Measure

2

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-2.mp4

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### If a Transversal Makes Equal ALternate Angles on two lines, the lines are Parallel

3

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-3.mp4

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### Two Lines are Paralles, if and only if, for any transversal, the corresponding Angles are Equal

4

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-4.mp4

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### The Diagonals of a Parallelogram bisect eachother

5

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-5.mp4

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### If Two Triangles are Similar, then their sides are Proportionate IN ORDER

6

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-6.mp4

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### If Three Parallel Lines cut off equal segments on some transversal, they will cut off equal segments on ALL transversals

7

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-7.mp4

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### In a Triangle ABC a line is drawn parallel to [AB] in a given ratio, it also cuts [AC] in the same ratio

8

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-8.mp4

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### If the Square of one side of a triangle equals the sum of the squares of the other two sides, the angle opposite the first side is 90 degrees

9

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-9.mp4

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### The Tangent to a Circle is at right angles to the radius at the point of contact.

10

https://s3.us-west-2.amazonaws.com/math.foundations/Theorem-10.mp4

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