Puzzle 113 - solution

 Since the lines oj and ol are radii, they are both equal to r. Which means the triangle is an isosceles triangle or equilateral triangle. Now since the sides oj and ol are equal, their opposite angles ojl and olj are equal. So each of these angles are (180 - 60)/2 = 60.

Which means the triangle ojl is an equilateral triangle. Which means its area is  √3 / 4 * r²  

 

Now area of sector jol is = πr² * 60/360 =   πr² /6