Triangles

Look at almost any built thing that has to stay standing — a roof truss, a bridge girder, the A-frame of a step ladder, the gable of a Kandyan-style house — and you will find triangles. Engineers trust triangles because their angles obey fixed, unbreakable rules. In this lesson you learn those rules: why the three angles of every triangle add to the same total, how an angle poking out of a triangle is secretly the sum of two others, and why a triangle with two equal sides always has two equal angles. These few facts unlock a huge slice of your O/L geometry paper, because almost every "find the angle" and "prove that" question leans on them.