SOLUTION: 20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term.

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Question 1086929: 20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term.
Found 2 solutions by Boreal, MathTherapy:
Answer by Boreal(15235)   (Show Source): You can put this solution on YOUR website!
a1=87 common difference is -15
a20=a1+(19)(-15)=87-285=-198
sum is n(a1+an)/2
=10(-111)
=-1110
87
72
57
42
27
12--positive add to 297
-3
-18
-33
-48
-63
-78
-93
-108
-123
-138
-153
-168
-183
-198--negative add to -1407
-1110 is sum.
Answer by MathTherapy(10552)   (Show Source): You can put this solution on YOUR website!

20.Find the sum of this series: 87 + 72 + 57 + 42 + …; 20th term.
Use the sum-of-an-AP formula: , with:

 = Sum of "n" terms (Unknown, in this case)

  = Number of terms (20, in this case)

 = 1st term (87, in this case)

  = Common difference (- 15, in this case)



Substitute these values and you should get:  as the sum. 

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