Explanation for the inner radius of the washer

Question

I'm learning integrals at uni and doing some practice questions right now. I have some trouble with 1 specific question and I was googling to see if I can find an explanation.

I came across this video that explains it but I'm still stuck at the same spot.

Here's the video with the same question.

https://www.youtube.com/watch?v=QX1pimVeJos

Why is the inner radius $3 + \sec x - 3$? I can't get my head around why that is the case. Any hint towards an answer would be great. Thanks. :)

Answer 1

We have given y = 3 + sec x as a complete radius of wheel. And as you can see inner radius = 3 + sec x - 3. Radius bounded by curve and y = 3.

I think from this image its clear to you.