How to you find the value of missing angels in a triangle?

Rule One: All triangles add up to 180º.

REMEMBER:

Equilateral Triangle: All sides are the same : 60 + 60 + 60 =180

Isosceles Triangle: Two sides the same : E.G. 50+ 50 + 80 = 180

Scalene Triangles: All sides different : E.G. 50+ 30 + 100

The method used depends on the situation:

▪ The "Z" method is simply if you have two parralel lines, and are joined by a straight line (Either in a Z shape or a _/‾ shape) the 'inside' angle at either side will be identical, and the 'outside' angle at either side will also be identical. This will rarely apply to triangles however.

▪ If you know 2 angles already, then you can use the fact all 3 angles of a triangle add together to equal 180. So, 180 subtracted by the other two angles will give you the third angle.

This is the simplest and most often needed.

The rest I am going to mention needs some work with forumlae. Depending what level of mathmatics you are working with, these may or may not be relevant at all.

Re-arranged this can be used as:

▪ If you know 2 side lengths but the known angle is not opposite either, you can use the "Cosine" rule to work out the third side. From there you can use the Sine rule to work out the other angles.

This rule can also be used to find any angle if you know all 3 sides, by re-arranging the forumla to:

You might find this helpful

https://www.mathsisfun.com/geometry/parallel-lines.html

alternate angles make a z shape

Co-interior angles make a C shape

and 

Corresponding angles make an F shape.

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If you scroll down on this one it willshow you the Z, C and the F very clearly :)

http://passyworldofmathematics.com/angles-and-parallel-lines/