SOLUTION: the fifth abd fifteenth term of an AP is 13 and 17 .find the sum of first 21 term

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Question 998451: the fifth abd fifteenth term of an AP is 13 and 17 .find the sum of first 21 term
Found 2 solutions by mananth, MathTherapy:
Answer by mananth(16946)   (Show Source): You can put this solution on YOUR website!
1 a + 4 d = 13 .............1
Total value
1 a + 14 d = 17 .............2
Eliminate d
multiply (1)by -7
Multiply (2) by 2
-7 a -28 d = -91
2 a + 28 d = 34
Add the two equations
-5 a = -57
/ -5
a = 11
plug value of a in (1)
1 a + 4 d = 13
11 + 4 d = 13
4 d = 13 -11
4 d = 2
d = 1/2

a= 11, d= 1/2 , n=15
Find


Plug in the values to get = 217.5
Answer by MathTherapy(10552)   (Show Source): You can put this solution on YOUR website!

the fifth abd fifteenth term of an AP is 13 and 17 .find the sum of first 21 term
Are you sure this is the problem?

If so, it involves a lot of fractions/decimals

You should get: Common difference, or  

You should get: First term, or  

You should get: Sum of first 21 terms, or  

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