\(\text{a) }-8< x< 8\)
\(\Rightarrow x\in\left\{\pm7;\pm6;\pm5;\pm4;\pm3;\pm2;\pm1;0\right\}\)
\(\left(\pm7\right)+...+\left(\pm1\right)+0=0\)
\(\text{b) }-6< x< 4\)
\(\Rightarrow x\in\left\{\pm3;\pm2;\pm1;0;-5;-4\right\}\)
\(\left(\pm3\right)+...+0+\left(-5\right)+\left(-4\right)=-9\)
\(\text{c) }-20< x< 21\)
\(\Rightarrow x\in\left\{\pm19;...\pm1;0;20\right\}\)
\(\Rightarrow\left(\pm19\right)+...+\left(\pm1\right)+0+20=20\)