In Circuits lab, we had to construct different types of operational amplifiers. The inverting and summing amplifiers produced similar output voltage to the calculated ones but were not negative. I was just wondering why they didn't reverse the sign in the experiment?

If you used a single positive supply,

a Hi would approach the value of the
power supply. A Lo will be approximately zero. The output can swing positive AND neg. only if you
used a p0s. AND a neg. supply. The Hi
would be almost equal to the pos. supply and the LO will be almost equal
to the neg. supply.