Suppose $y^{\text{'}}=f(x,y)=\frac{xy}{cos(x)}.$

a$.\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{y}}=$ help $($formulas$)$

b$.$ Since the function $f(x,y)$ is $$ continuous at the point $(0,0),$ the partial derivative $\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{y}}$ and is $$ at and near the point $(0,0),$ the solution to $y^{\text{'}}=f(x,y)$ exists and is unique near $y(0)=0$