Intercept Theorem Calculator (Thales) with Converse
Find a missing length with the intercept theorem, also called the basic proportionality or side splitter theorem, or test parallel lines with its converse.
Nested triangles: A, M, B and A, N, C are collinear in that order. Crossed (hourglass) figure: name the smaller triangle AMN; the calculation is the same.
The selected length is calculated as soon as its partner length and one complete ratio (for example AM and AB) are entered.
Enter the partner length of the unknown and one complete ratio of corresponding lengths. Each length of the small triangle AMN must be shorter than the matching length of the large triangle ABC.
Enter AM, AB, AN and AC with AM < AB and AN < AC.