Summing amplifier - output voltage and power supply
If you have an inverting summing amplifier with 10K+10K and Rf 100K, then your signal would be -30V to 10V, except the amplifier will clip the output signal at (well, in reality, before) it hits the supply rails. The amplifier cannot drive its output voltage beyond the rails (how close it can get with any kind of load depends on the type of amplifier). 

A fair approximation for your purposes would be that it would be: 

Vout = max(\$ -20sin(200\pi t) -10 , -10\$) 

In reality it would probably clip a bit before that (so it would also likely clip on the positive peaks) and could (for some op-amps) take a noticeable amount of time (even at 100Hz) to recover from the clipping.