We need to find the point of intersection of the curves f1(x) = cos-1(cos x) and f2(x) = 10 - x/10 in the domain [0, 4π].
f1(x) is a period function with period 2π and f2(x) is a straight line plotting both graphs
Therefore, A, B and C are the points of intersection of both curves which obviously satisfy the given equations, hence there are three such points.