The question I am stuck on is asking to find the volume of a square frustum with has height $h$, upper side length $l$, and lower side length $k$. I have to solve it using a definite integral, so I can't create it into a full triangle and chop off the top using similar triangles; using geometry/similar triangles is kinda out.
This is what I have so far. I believe I need to take square cross section and the square formula would be $a(x)= f(x)^2$. However, I do not know how to figure out the height. If someone could help me out in the right direction that would be great! Thank you.
Can you write down a linear expression, $f(x)$, such that $f(0)=k$ and $f(h)=l$? If you can do that, then you could integrate the square of $f(x)$ from $x=0$ to $x=h$.