x0) being impossible to find so it can be dropped. My lecture notes also say f(x) = f(x0) + integral of f'(t)dt with upper bound x and lower bound x0. I'm just confused as to which is the tangent line approximation because they seem to be different equations. If the integral is evaluated on the 2nd one it gives f(x)-f(x0) but the 2nd term for the first equation (dropping R1(x;x0) gives xf'(x0)-x0f'(x0) which seems to be a different term to me. So I'm wondering which is the tangent line equation (or if the tangent line equation has more than 1 unique form) or how the second one equals the first if they are equal.