Find S25 for -7+ -2 + 3 + 8 +…

Question

rational · Answer

see any pattern ?

anonymous · Answer

+5

rational · Answer

yes, what do you know about arithmetic series ?
know how to find partial sums ?

anonymous · Answer

well i got the answer 1,325 but the way i did it took longer then the way it should with a formula.. so i really just need the quick way to do it

rational · Answer

$$\large S_n =  \dfrac{n}{2}\left[2a + (n-1)d \right]$$

familiar with this formula ?

anonymous · Answer

ive seen a dif but never saw this one

rational · Answer

btw 1325 is correct http://www.wolframalpha.com/input/?i=25%2F2%282%28-7%29+%2B+%2825-1%29*5%29

rational · Answer

which formula have u seen before ?

anonymous · Answer

$$Sn=\frac{ a1(1-r^n) }{ 1-r }$$

anonymous · Answer

but i understand the one you showed me, so thank you very much!

rational · Answer

$$Sn=\frac{ a1(1-r^n) }{ 1-r }$$

this is a partial sum formula for GEOMETRIC series, 
where the next term is a multiple of some constant ratio $r$

rational · Answer

but the given series is ARITHMETIC here as the difference between two consecutive terms is same So you need to use the arithmetic series formula

anonymous · Answer

ohh ok thank you!