Using differentials to estimate change

Question

A person's BMI is given by I(W,H) = W/H^2, where W is the person's weight in kilograms and H is height in meters. Suppose I am 1.65 meters tall and weigh 54.5 kilograms. After a month, I become 1.55 meters tall and weigh 59.5 kilograms. Use differentials to estimate the change in my BMI. 

I'm not sure what I'm supposed to do. 

Thanks. 

Answer 1

The gradient of I(W,H) is going to be the sum of the partial derivatives of I with respect to its independent variables. You can represent this as a set of finite difference equations. Then plug in your H_0 and H_1, and W_0 and W_1 values to estimate the gradient of your BMI.