def quadratic_interp(a,f,x):
    """Compute at quadratic interpolant
    Args:
        a: array of the 3 points
        f: array of the value of f(a) at the 3 points
    Returns:
        The value of the linear interpolant at x
    """
    answer = (x-a[1])*(x-a[2])/(a[0]-a[1])/(a[0]-a[2])*f[0] 
    answer += (x-a[0])*(x-a[2])/(a[1]-a[0])/(a[1]-a[2])*f[1] 
    answer += (x-a[0])*(x-a[1])/(a[2]-a[0])/(a[2]-a[1])*f[2] 
    return answer