# Course 1 - Week 2 - Exercise 5 - propagate

The first step is to figure out why your dw values are the wrong shape. My guess is that you are using “elementwise” multiply instead of dot product in the formula for dw. Here’s the math formula that you need to write the code for:

dw = \displaystyle \frac {1}{m} X \cdot (A - Y)^T

The key point is that the operation between X and (A - Y)^T is a dot product style matrix multiply, not an “elementwise” multiply.

Look at the dimensions of the inputs to the first test case for propagate:

w is 2 x 1
X is 2 x 3
Y is 1 x 3

That means A will also be 1 x 3. So look at the dimensions on that dot product:

X is 2 x 3 and (A - Y)^T will be 3 x 1. So the result will be 2 x 1, which is the same as the shape of w. But if you use * instead of np.dot and leave out the transpose, then you will end up with a 2 x 3 output.

Here’s a thread about how to figure out when to use elementwise multiply and when to use dot product. Please have a look and see if that helps.