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October 20, 2014

October 20, 2014

Posted by **Mae** on Sunday, January 4, 2009 at 10:58pm.

Can someone please explain how to do this? I don't know which formula to use. TIA

- Math-Gr.11 urgent -
**Reiny**, Sunday, January 4, 2009 at 11:31pmYou know t(10) = a+9d

a+9d = 34

or a = 34-9d

S(20) = 20/2[2a + 19d]

710 = 10(2a + 19d)

71 = 2a + 19d

71 = 2(34-9d) + 19d

71 = 68 - 18d + 19d

3 = d

a = 34-27 = 7

then t(25) = a+24d = 7 + 24(3) = 79

- Math-Gr.11 urgent -
**Mae**, Sunday, January 4, 2009 at 11:47pmfor t(10)= a+9d .. where did you get the 9 from? & can you explain how you got 710? thank you

- Math-Gr.11 urgent -
**Reiny**, Monday, January 5, 2009 at 12:01amyou should know that

t(n) = a + d(n-1) one of the most basic formulas in the study of sequences

and you gave me 710 in the question!

- Math-Gr.11 urgent -
**Mae**, Monday, January 5, 2009 at 12:04amoh sorry sorry. i never got that you simplified it. thanks

- Math-Gr.11 urgent -
**Jake**, Tuesday, October 20, 2009 at 5:07pmhi, 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

- Math-Gr.11 urgent -
**A. S**, Sunday, November 11, 2012 at 1:36amThis was GREAT

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