Find the sum of the following AP's.
(i) -37, -33, -29, .... to 12 terms
(ii) 115,112,110, ... to 11 terms.
(i) -37, -33, -29 ... to 12 terms
First term = a = -37
Common difference = d = -33 - (-37) = 4
n = 12
Applying formula, Sn=n2(2a+ (n−1) d) to find sum of n terms of AP, we get
Sn=122[−74+ (12−1) (4)] =6[−74+44] =6[−30] =−180
(ii) 115,112,110... to 11 terms
First tern = a = 115
Common difference = d = 112−115= -3
Applying formula, Sn=n2(2a+ (n−1) d) to find sum of n terms of AP, we get
Sn=112[2(115)+ (11−1) (-3)] =112[230 - 30] =11/2[200]= 1100