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
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
