Hi

For the parabola y = 0.5 (x^2 +1), why is the equation of the tangent at point P (a,b): xa = y + b -1 (in the textbook)?

I tried differentiating the parabola equation to find the gradient (a) and then using y-y1 = m(x-x1) formula but it doesnt turn out to be that...

Please help?!?