@StMauricesMaths Consider the line AB and a point C. Construct line DE thru C parallel to AB with D nearer A and E nearer B. Now ∠DCA=∠CAB and ∠ECB=∠ABC. By definition angles on a line sum to 180* so ∠DCA+∠ECB+∠BCA=180* therefore ∠ABC+∠BCA+∠BAC=180*... I think🤔