Thales's intercept theorem

The intercept theorem (Thales's theorem) states that parallel lines crossing the arms of an angle cut off proportional segments on them:

$\frac{a}{b}=\frac{c}{d}$

where $a, b$ are segments on one arm and $c, d$ the corresponding segments on the other. Knowing three of the four segments, you find the fourth by cross-multiplication ($a\cdot d=b\cdot c$).

It underlies, among other things, dividing a segment in a given ratio, finding inaccessible distances and proving triangle similarity. Leave one field blank and the calculator computes the missing segment.