Posted by **Crystal** on Tuesday, January 8, 2013 at 6:02pm.

I'm learning series and sequences (grade 11).

Please check that my steps show I understand what I'm doing/the concept and my answer as well:

5. The consecutive terms of an arithmetic sequences are 3.6, y, 8.2. Find the value of y.

This seems to be a sequence question but I used the formula for series.... Is there a better way to solve this question???

S3 = 3/2 (3.6+8.2)

=3/2(11.8)

=17.7

17.7 = 3/2(2 • 3.6 + (3-1)d)

17.7 = 3/2(7.2+2d)

17.7 = 10.8 + 3d

17.7 - 10.8 = 3d

6.9 = 3d

2.3 = d

So y = 5.9

I feel like I'm not doing this properly ... Please help! I have a quiz tomorrow on sequences ):

I also used the same method for this question:

Insert 3 evenly spaced numbers between -2 and 10. My final answer was 3 = d, so : -2, 1, 4, 7, 10.

What is the proper way to solve these questions? I don't think I'll be able to use series formulas on the quiz tomorrow which is only on sequences!

Also, I'm stuck on this question:

Find the 10th term of the arithmetic sequence where the first term is 5 and the 4th term is 17.

How would you solve this question?

Thank you so much in advance!

- Math -
**Reiny**, Tuesday, January 8, 2013 at 7:28pm
Consecutive numbers of an arithmetic have a common difference between them, that is ...

y - 3.6 = 8.2-y

2y = 11.8

y = 5.9

or

y must be "average" of the two other numbers

y = (3.6+8.2)/2 = 5.9

You sure went about it the long way.

For splacing 3 evenly spaced numbers between -2 and 10

-2, -, -, -, 10

then -2 must be the first term or a = -2

10 is th 5th term

term(5) = a + 4d

a+4d=10

-2+4d=10

4d=12

d=3

Again, your answer is correct

last question:

given: fifth term is 5 ---> a+4d = 5

4th term is 17 ----> a+3d = 17

subtract the two equations

d = -12

then in a+3d = 17

a + 3(-12) = 17

a= 17 + 36 = 53

term(10) = a+9d = 53 + 9(-12) = -55

check:

if a=53, d = -12, the first few terms are

53 41 29 17 5 ...

our answer is correct

- Math -
**Crystal**, Tuesday, January 8, 2013 at 7:40pm
Thank you so much Reiny!!

- Math -
**yo**, Tuesday, July 1, 2014 at 11:33pm
d doesnt = 12 because to subtract the 2 equations you would get 12d=18 then you whould have to devide and find that d=1.5

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