\(\left(-30\right)+\left(-29\right)+-...+48+49+50\)
=\(\left(\left(-30\right)+30\right)+\left(\left(-29\right)+29\right)+\left(\left(-1\right)+1\right)+31+32+-49+50\)
\(=31+32+--..+49+50\) = \(\left(31+49\right)\left(32+48\right)+--..\left(40+40\right)+50\)
\(\left(80.9\right)+50=720+50=770\)