## anonymous one year ago I don't know why I am getting the wrong answer, please help! 16. The first term of an arithmetic progression is 6 and the fifth term is 12. The progression has n terms and the sum of all the terms is 90. Find the value of n. I will post a picture of my working out.

1. anonymous

2. anonymous

3. Nnesha

i got 8 !! :D now wait a sec i'll check ur work

4. Nnesha

http://prntscr.com/8ig2duis it 45/n ??

5. anonymous

The screenshot didn't work

6. anonymous

But in the beginning I did get 45/n

7. Nnesha

okay so at left side is it 45/n it's a 2nd row

8. Nnesha

how did you get that ?

9. anonymous

Oooh...I combined 1/2 n and 90. not like terms so I cant do that right?

10. Nnesha

it's $\frac{ 3 }{ 2}n = \frac{3n}{2}$ n isn't at the denominator

11. Nnesha

ye first combine 12-3/2

12. Nnesha

$\huge\rm 90=\frac{ n(\color{Red}{12}+\frac{ 3 }{ 2 }n-\color{Red}{\frac{ 3 }{ 2} })}{ 2 }$ like terms

13. anonymous

Ok. I will try again quickly.

14. Nnesha

okay :=)

15. anonymous

I got 7.46, Did I lose some accuracy somewhere or is that what I am meant to get?

16. Nnesha

yes that's not correct

17. anonymous

Awwww. Oh well. Ok should i post my working out again?

18. Nnesha

yes

19. welshfella

a + 4d - a = 4d = 12 - 6 = 6 so d = 3/2

20. Nnesha

first multiply both sides by 2 $\huge\rm 90 · 2=\frac{ n(\color{Red}{12}+\frac{ 3 }{ 2 }n-\color{Red}{\frac{ 3 }{ 2} })}{ 2 }· 2$ combine 12-3/2 and then distribute parentheses by n

21. anonymous

22. anonymous

the n=9 is from the other equation

23. anonymous

Oh. I think I forgot that its over 2

24. Nnesha

no you didn't you distributed the parentheses by (1/2)n

25. anonymous

Yes?

26. anonymous

I'm sorry, I don't really understand whaI did wrong?

27. Nnesha

i know what happened t o12.25 ?? check ur 6th and 7th row

28. anonymous

I was making a perfect square

29. Nnesha

w|dw:1442760681511:dw| you should add (b/2)^2 to both sides

30. Nnesha

it's n^2 ** typo

31. anonymous

Oh!! I forget to do that a lot >:/ Ok, third time lucky. Trying again.

32. anonymous

Aaaaand I am finally getting it right!

33. Nnesha

Yahooo!!

34. anonymous

Thank you for your patience and for helping me so much! :D

35. Nnesha

my pleasure &GREAT JOB!! i like how u posted ur work :=) that was super cool!

36. Nnesha

good luck!!

37. anonymous

Thanks!

38. Nnesha

o^_^o