- #1
mattyh3
- 25
- 0
Homework Statement
How many terms of the AP, 12, 16, 20, 24 . . . . . must be taken for the sum to
equal 408?
Solution
For this series, a = 12, d = 4 and Sn = 408.
its asking me to solve for n and gives me the answer to n as 12 at the end but when i try to calculate it myself i don't get 12 and it only states limited steps and i a bit unsure on how to transpose the equation to make n the subject so it comes out with the answer 12
Homework Equations
sn= n/2 (2a + (n-1)d)
then its has
408=n/2 (2 x 12 +(n-1) x 4)
but am not to great at transposing and can't seem to make n the subject and get the answer of 12 can anyone help with this as its only an example on my lesson book i just want to understand how its transposing to get n = 12
Solving this for n gives n = 12. The other root of the equation is negative and
is rejected as impractical.