A triangle has all three angles summing up to 180. If two of the sides measure 40 degrees and 50 degrees, then the third side measures as follows;
[tex]\begin{gathered} 40+50+A=180 \\ A=180-40-50 \\ A=90 \end{gathered}[/tex]The third angle, which is labelled as angle A measures 90 degrees, and that means the triangle is a "right angled triangle."
