Further Maths Edexcel Question (Series) Watch

Davidstbridge
Badges: 0
#1
Report Thread starter 9 years ago
#1
\sum_{r=1}^{20}\frac{1}{(r+3)(r+  6)}

These types of questions I really cannot understand. I've tried multiplying out the brackets and then putting each term into n, then plugging in 20 and all dividing by 1.

This does not give me the right answer.

I've also tried splitting the fraction and using the method of differences, to no avail.

I'm probably missing something really obvious but its annoying me as its with every question in a fraction.

Help would be much appreciated =]
0
quote
reply
ghostwalker
  • Study Helper
Badges: 15
#2
Report 9 years ago
#2
If you split it, as per partial fractions, you can write it as the difference of two sums, most of whose terms will cancel out.
quote
reply
benwellsday
Badges: 12
Rep:
?
#3
Report 9 years ago
#3
 \frac{1}{(r+3)(r+6)} = \frac{1}{3} (\frac{1}{r+3} - \frac{1}{r+6}) then use the differences method. You might need to write down a few initial results like r = 1, r =2 etc before you spot what's cancelling.
0
quote
reply
Adjective
Badges: 12
Rep:
?
#4
Report 9 years ago
#4
The method of differences should work (having split it into partial fractions).
0
quote
reply
Davidstbridge
Badges: 0
#5
Report Thread starter 9 years ago
#5
yeah i've tried that already, just made sure again now. nothing actually cancel's out i'm pretty sure. tried up to 5, and only 1 term could be cancelled out. At a loose end here...
0
quote
reply
benwellsday
Badges: 12
Rep:
?
#6
Report 9 years ago
#6
Stuff does cancel, after 3 of them...
1/3 - 1/6
+
1/4 - 1/7
+
1/5 - 1/8
+
1/6 - 1/9
+
1/7 - 1/10
...
...
1/23 - 1/26
0
quote
reply
Davidstbridge
Badges: 0
#7
Report Thread starter 9 years ago
#7
ha okay sorry, my stubborness to give up easily messed me up on this question. Thanks for your patience guys, got there in the end =]
0
quote
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • University of Lincoln
    Mini Open Day at the Brayford Campus Undergraduate
    Wed, 19 Dec '18
  • University of East Anglia
    UEA Mini Open Day Undergraduate
    Fri, 4 Jan '19
  • Bournemouth University
    Undergraduate Mini Open Day Undergraduate
    Wed, 9 Jan '19

Were you ever put in isolation at school?

Yes (170)
27.07%
No (458)
72.93%

Watched Threads

View All