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Mathematics 84 Online
OpenStudy (anonymous):

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.

OpenStudy (anonymous):

OpenStudy (anonymous):

The correct answer is 8

Nnesha (nnesha):

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

Nnesha (nnesha):

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

OpenStudy (anonymous):

The screenshot didn't work

OpenStudy (anonymous):

But in the beginning I did get 45/n

Nnesha (nnesha):

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

Nnesha (nnesha):

how did you get that ?

OpenStudy (anonymous):

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

Nnesha (nnesha):

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

Nnesha (nnesha):

ye first combine 12-3/2

Nnesha (nnesha):

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

OpenStudy (anonymous):

Ok. I will try again quickly.

Nnesha (nnesha):

okay :=)

OpenStudy (anonymous):

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

Nnesha (nnesha):

yes that's not correct

OpenStudy (anonymous):

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

Nnesha (nnesha):

yes

OpenStudy (welshfella):

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

Nnesha (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

OpenStudy (anonymous):

OpenStudy (anonymous):

the n=9 is from the other equation

OpenStudy (anonymous):

Oh. I think I forgot that its over 2

Nnesha (nnesha):

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

OpenStudy (anonymous):

Yes?

OpenStudy (anonymous):

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

Nnesha (nnesha):

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

OpenStudy (anonymous):

I was making a perfect square

Nnesha (nnesha):

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

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