NatalieLove Group Title What is the sum of the arithmetic sequence 137, 125, 113 …, if there are 38 terms? 2 years ago 2 years ago

1. myininaya

Do you see that the numbers are increasing by 8?

2. NatalieLove

yes

3. mariomintchev

i would do it the old school way and keep subtracting 12 until you get 38 terms, then add lol

4. myininaya

$\sum_{i=0}^{37}(137+8i)=137+8(0)+\sum_{i=1}^{37}(137+8i)$ Now use $\sum_{i=1}^{n}i=\frac{n(n+1)}{2} \text{ and } \sum_{i=1}^{n}c=cn$

5. NatalieLove

Im still not getting the correct answer

6. Zarkon

whay are people using d=8

7. Zarkon

*why

8. NatalieLove

i dont know im gonna guess

9. FoolForMath

Whenever you need to find the sum of an arithmetic series use this formula: $\frac n 2 \left( 2a+ (n-1) \right)\times d) \tag{1}$ a= first term n= number of terms d= common difference Here, $$n= 38, a = 137$$ and $$d = -12$$. Substituting these in $$(1)$$, and assuming my algebra is right you answer should be $$-3230$$

10. NatalieLove

I know but the answer does not show up in the multiple choose

11. FoolForMath

@NatalieLove: Revert if you need help in proving that formula.

12. NatalieLove

We are doing the same steps the test its incorrect. It does not show up as a choose Thank You

13. FoolForMath