Constructing a triangle without a protractor Place compass point at point R, and draw an arc using compass opened to 8 cm, so as to cross the first arc.

Click to see a step-by-step slideshow. STEP 1: This is what to do when you know the length of 1 side and the 2 angles at each end of it: here 8 cm, 40° and 50°. Draw an 8 cm line using a pencil and ...