f(x) = (sqrt(x) - 1.0) / (x - 1.0)
a = 2 (slider from 1 to 2, step 0.01)
A: (a, f(a))
testo1: "\frac{\sqrt{a}-1}{a-1}={\frac{\sqrt{" + a + "}" + (-1.0) + "}{" + a + "-1}=" + (y(A))