Hello,
First , number of numbers: (131-(-4))/9 +1=15+1=16
[(last number- first number)/ (2th number- 1th number) +1
[tex] \sum_{i=1}^{16}{(-13+i*9) =-13 \sum_{i=1}^{16}{1} + 9*\sum_{i=1}^{16}{i}
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[tex]=-13*16+9*\frac{16*(1+16}{2}=-208+9*136=1016
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