Label the angle formed between the intersecting lines with the uppercase letter C, and measure angle C using the protractor.
Label the lines that intersect to form angle C with lowercase letters a and b.
Join the free ends of the lines a and b with a third pencil line so that a triangle is formed. Label this third line with the lowercase letter c. Measure the length of c.
Label the angle opposite to line b as angle B, and the angle opposite to line a as angle A. Measure angles A and B using the protractor.
Find the unknown side lengths a and b using this relationship: a/(sin A) = b/(sin B) = c/(sin C). First calculate c/(sin C) as both length c and angle C are known. Equate c/(sin C) to a/(sin A) to find the length a. Equate c/(sin C) to b/(sin B) to find the length b. For example, start with an angle (C) of 60 degrees and draw side c with length 10 cm, so that the triangle you create has measured angles A = 45 degrees, and B = 75 degrees. Side length a = (sin A) x (c)/(sin C) = (sin 45) x (10)/(sin 60) = 8.16 cm. Side length b = (sin B) x (c)/(sin C) = (sin 75) x (10)/(sin 60) = 11.15 cm.