I am studying the sound wave equation deducted by Feynman in his lectures. In section 47-3:
P0 + Pe = f(d0 + de) = f(d0) + de f'(d0)
Where f'(d0) stands for the derivative of f(d) evaluated at d=d0. Also, de is very small.
I do not understand the second step of the equality. Can anyone help...