f_1(x) = x^(2) - (4 * x) + 1
f_2(x) = 1 / ((2 * x) - 1)^(2)
f_3(x) = 1 / ((10 * (sin(x))^(2)) - 3)
f_4(x) = 1 / ((10 * (sin(x))^(2)) - 13)