Should the plus sign in this formula be a minus sign?

This screenshot is from the Gradient Checking assignment from Week 1:

The plus sign in the denominator should be a minus, right?

Hey @Teodora_Petkova,
Welcome to the community. The plus sign in the denominator is correct in the formula. For finding the relative difference between any 2 vectors, we take the L2 norm of their difference over the sum of L2 norms of the individual vectors.

In fact we can easily see why the minus sign in the denominator would be incorrect. Consider 2 equal vectors, A and B, i.e., they have the same magnitude and direction. Trivially, the relative difference between them should be 0, but if we consider minus sign in the denominator, then the relative difference would be of the form (0 / 0). I hope this helps.