## 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 Group Title

Do you see that the numbers are increasing by 8?

2. NatalieLove Group Title

yes

3. mariomintchev Group Title

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

4. myininaya Group Title

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

5. NatalieLove Group Title

Im still not getting the correct answer

6. Zarkon Group Title

whay are people using d=8

7. Zarkon Group Title

*why

8. NatalieLove Group Title

i dont know im gonna guess

9. FoolForMath Group Title

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 Group Title

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

11. FoolForMath Group Title

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

12. NatalieLove Group Title

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

13. FoolForMath Group Title

Glad to help :)

14. myininaya Group Title

omg lol I said increasing by 8 and then continue to think the thingy was increasing by 8 sometimes i wonder about myself lol