| − | where F(y) is a "constant of integration". (In reality, F(y) is a [[function]], but when we differentiate it with respect t, we treat y as constant and so its [[derivative]] becomes zero.) In order to solve for F(y), we differentiate φ(y,t) with respect to the other variable, in this case y, and set the result equal to N(t,y). | + | where F(y) is a "constant of integration". (In reality, F(y) is a [[function]], but when we differentiate it with respect t, we treat y as constant and so its [[Derivative_(calculus)|derivative]] becomes zero.) In order to solve for F(y), we differentiate φ(y,t) with respect to the other variable, in this case y, and set the result equal to N(t,y). |