Hey for this course, in 3:30, I don’t understand two things:

- Why one edge is - delta cos(X)
- Why another edge is delta sin(X)

Anyone can help?

Hey for this course, in 3:30, I don’t understand two things:

- Why one edge is - delta cos(X)
- Why another edge is delta sin(X)

Anyone can help?

Here is the screenshot:

The first thing is to realize that the definitions are:

\Delta sin(x) = sin(x + \Delta x) - sin(x)

\Delta cos(x) = cos(x + \Delta x) - cos(x)

Now look at the diagram. The length of the bottom edge of the small orange triangle is:

cos(x) - cos(x + \Delta x)

So that is equal to

-\Delta cos(x)

Now apply the same logic to the left edge of the orange triangle. The length is:

sin(x + \Delta x) - sin(x)