## anonymous one year ago Find the sum of a finite arithmetic sequence from n = 1 to n = 15, using the expression 2n + 5. 315 215 32 35

1. anonymous

@dan815

2. anonymous

@welshfella

3. anonymous

4. anonymous

what is the first term, when $$n=1$$?

5. anonymous

I dont know

6. anonymous

really?

7. anonymous

replace $$n$$ by $$1$$ in $$2n+5$$

8. anonymous

so its 7?

9. anonymous

yes now hold that number

10. anonymous

what is $$2n+5$$ if $$n=15$$?

11. anonymous

35?

12. anonymous

yes

13. anonymous

14. anonymous

formula you are going to use is $\frac{n}{2}(a_1+a_n)$ you have $$a_1=7,a_{15}=35$$

15. anonymous

what is$$7+35$$?

16. anonymous

Its 42

17. anonymous

ok so we are at $\frac{15}{2}(42)$

18. anonymous

divide 42 by 2, then multiply the result by 15

19. anonymous

Its 315