# How do you find the value of x in a pair of similar polygons?

Asked on by raekwon

mcetner | High School Teacher | (Level 1) Adjunct Educator

Posted on

If x is a side, set up a proportion. You are usually given four pieces of information all together (maybe 3 numbers and x, maybe 2 numbers and two expressions of x). Corresponding sides go in the same fractions. For example, say triangles ABC and DEF are similar. Then you know sides AB/DE = BC/EF.

If x is an angle, corresponding angles are equal. So, for triangles ABC and DEF, angleABC = angleDEF.

revolution | College Teacher | (Level 1) Valedictorian

Posted on

To prove similarity, corresponding angles must be the same, and also the ratio of the corresponding sides must be proportional, meaning a/b and c/d must get the same value.

So, if x is an angle, you will get information like 2 known angle values and one unknown angle value, or all three known angle values.  angle a= angle b

If x is an length, you will get three sides of any polygon being proportionate to the corresponding sides of another similar polygon or only two pairs of corresponding sides  a/b=c/d

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