hi so basically my question is derivation of derivation of the quadratic case of the terminal velocity I have attached the pictures in picture 1 I don't know I think the book has error in his derivation @ equation 2.4.13 shouldn't it be + instead of negative ?? & in second picture their is something I don't understand two stuff acctually first is in equation 2.4.15 how did he come up with 1/2 * dv^2 / dy? and my second question is du/dy = -1/vt^2 * dv^2/dy how did he come up with dv^2/dy ?? thanks alot.