SOLUTION: Find the sum of the first 30 terms of the arithmetic sequence : 10,5,0,-5

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Question 1091746: Find the sum of the first 30 terms of the arithmetic sequence : 10,5,0,-5
Found 2 solutions by Boreal, MathTherapy:
Answer by Boreal(15235) (Show Source):
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sum is (n/2)(a1+an)
a1=10
an is a1+(n-1)d
a1=10
n-1=29
d=-5
a30=10+29(-5)=-135
S=(30/2)(10-135)=15(-125)=-1875
10
5
0
+ terms are +15
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
-95
-100
-105
-110
-115
-120
-125
-130
-135

Answer by MathTherapy(10552) (Show Source):
You can put this solution on YOUR website!
Find the sum of the first 30 terms of the arithmetic sequence : 10,5,0,-5

Use the formula for the sum of an AP when the number of terms, the 1st term, and the common difference are known, which is:
----- Substituting 30 for n, 10 for , and - 5 for d

Sum of this AP, or