How can you use triangles to find the area of circle?

Answer 1

If you draw your triangles using the centre of the circle as one vertex and two more on the circumference, the area of the circle is approximately equal to the sum of the areas of the triangles; the smaller you make the triangles, the more accurate your result will be.

Answer 2

You can divide up a circle into triangles (more precisely, approximate it with triangles), or with rectangles, in several ways. One simple and symmetric way that uses triangles is to trace several radii - if they are close to one another, the resulting circular sections are close to triangles.