\(\left(-30\right)+\left(-29\right)+...+48+49+50\)
\(=\left[\left(-30\right)+30\right]+\left[\left(-29\right)+29\right]+-.+\left[\left(-1\right)+1\right]+31+32+...+50\)\(=0+0+0+-.+31+32+-.+50\)
\(=\left(\dfrac{50-31}{1}+1\right):2\left(31+50\right)=10.81=810\)