Question

find s11 for 3+1+(-1)+(-3)+....
-77
-75
-73
-71

Answer 1

a= 3
r = U2-U1 = U4-U3= 1-3 = -3-(-1) = -2

so s 11 = n/2 (2a+(n-1)r) = 11/2 (2*3+ (11-1)-2) = 5.5 (6-20)=5.5 * (-14) = -77

Answer 2

s6 = -7
s11 = (s6)*11 = -7*11 = -77