how do you find the sum of this arithmetic series?

Question

1. -27+(-17)+(-7)+....+43
2. 1 + 4 + 7 +...+142
3. 400+394+388+...+(-32)
4. the first 20 terms of the series 13+20+27+...

Answer 1

S = n(t1+tn)/2
S = 8(-27+43)/2 = 64

3n-2 = 142 so n = 48
S = 48(1+142)/2 = 3432

S= 73(400 -32)/2 = 13,432

S = n(2t1+(n-1)d)/2
S = 20(2*13 +(20-1)7)/2 = 1590