Posted by Mae on Sunday, January 4, 2009 at 10:58pm.
The 10th term of an arithmetic series is 34, and the sum of the first 20 terms is 710. Determine the 25th term.
Can someone please explain how to do this? I don't know which formula to use. TIA

MathGr.11 urgent  Reiny, Sunday, January 4, 2009 at 11:31pm
You know t(10) = a+9d
a+9d = 34
or a = 349d
S(20) = 20/2[2a + 19d]
710 = 10(2a + 19d)
71 = 2a + 19d
71 = 2(349d) + 19d
71 = 68  18d + 19d
3 = d
a = 3427 = 7
then t(25) = a+24d = 7 + 24(3) = 79 
MathGr.11 urgent  Mae, Sunday, January 4, 2009 at 11:47pm
for t(10)= a+9d .. where did you get the 9 from? & can you explain how you got 710? thank you

MathGr.11 urgent  Reiny, Monday, January 5, 2009 at 12:01am
you should know that
t(n) = a + d(n1) one of the most basic formulas in the study of sequences
and you gave me 710 in the question! 
MathGr.11 urgent  Mae, Monday, January 5, 2009 at 12:04am
oh sorry sorry. i never got that you simplified it. thanks

MathGr.11 urgent  Jake, Tuesday, October 20, 2009 at 5:07pm
hi, i was wondering where you got:
20/2[2a + 19d] from, im in yr 11 but want to start revising a level because i want to get a phd in maths, so i just wanted to know where you got it from, if you could answer this, then thanks 
MathGr.11 urgent  A. S, Sunday, November 11, 2012 at 1:36am
This was GREAT