List the side lengths of △STU in order from smallest to largest, given that m∠U=45°, m∠S=80°, and m∠T=55°.

In a triangle, the size of an angle is directly proportional to the length of its opposite side. This means that the larger the angle, the longer the side opposite it.

Given that m∠U=45°, m∠T=55°, and m∠S=80°, we can say that:

1. The smallest angle is ∠U, so the side opposite it, which is ST, is the smallest side.
2. The next smallest angle is ∠T, so the side opposite it, which is SU, is the next smallest side.
3. The largest angle is ∠S, so the side opposite it, which is TU, is the largest side.

So, the side lengths of △STU in order from smallest to largest are ST, SU, and TU.