Question 1191995
sum=(n/2)(2a+(n-1)d)
nth term =a1+(n-1)d
a. n15=7+(15-1)2=35
Sum is (15/2)(14+14*2)=315
7+9+11+13+15+17+19+21+23+25+27+29+31+33+35=315
-
a1=-9, d=3, sum=66
66=(n/2)(-18+(n-1)3)=(n/2)(-18+3n-3)=(n/2)(3n-21)
multiply by 2 to clear fractions
132=3n^2-21n
n^2-7n-44=0 dividing by 3 and rearranging.
(n-11)(n+4)=0
n=11 terms only positive root.
-9,-6,-3,0,3,6,9,12,15,18,21
-
This is n(n+1) where n=25
The answer is 650.