Hello,
let
t=(-4)+(-12)+(-20)+(-28)+(-36)+....=-(4+12+20+28+36+...)=-s
\(a_{1}=4=-4+1*8\)
\(a_{2}=12=-4+2*8\)
\(a_{3}=20=-4+3*8\)
\(a_{4}=28=-4+4*8\)
...
\(a_{18}=?=-4+18*8=140\)
\(a_{i}=-4+i*8\)
\(s=\sum_{i=1}^{18} (-4+i*8)=\sum_{i=1}^{18} (-4)+\ 8*\sum_{i=1}^{18}(i)\\\\=(-4)*18+8*(1+2+3+4...+18)\\\\=-72+\frac{(1+18)*18}{2}\\\\=-72+171=99\)
\(\boxed{\ t=-99}\)
