More detailed error propagation formulas for Assignment 4

I think formulas in section Error propagation (analytical) were a bit unclear.
For example, the uncertainty formula for multiplication was meant for two independent variables, and thus is not suitable for verifying assignment 1, with an $r^2$ in the denominator.
My suggestion is to specify that the formulas are for independent variables, or use this formula [https://wikimedia.org/api/rest_v1/media/math/render/svg/7a7686692e58c0857185c50e2146a153449bade3](url). It requires to take partial derivative of each variable involved, avoiding the issue above.

Find the notebook section titled “Error propagation (analytical)” and compare its multiplication uncertainty formula with the linked partial-derivative reference. Clarify the independent-variable assumption or replace the formula as appropriate; done means the formulas clearly support the assignment context, including Assignment 1.

Written by the indexing model from the issue text.