This discussion is closed.
john !!
Badges: 14
Rep:
?
#1
Report Thread starter 15 years ago
#1
please make solutions in white --->like this<---
if you can do this it feels very satisfying because there's a very subtle solution
ant try not to cheat by looking at others!

(I hope I've typed it out correctly)
Attached files
0
dvs
Badges: 10
Rep:
?
#2
Report 15 years ago
#2
Like this you mean? (Or do you want it simplified further or solved?)
Attached files
0
Jonny W
Badges: 8
Rep:
?
#3
Report 15 years ago
#3
dvs: shouldn't that be (n^2 + n)^(1/3) rather than (n^2 + 2n)^(1/3) ?
0
dvs
Badges: 10
Rep:
?
#4
Report 15 years ago
#4
Yes, it should. I mistyped it!
0
john !!
Badges: 14
Rep:
?
#5
Report Thread starter 15 years ago
#5
I believe it can be simplifies further if I have types it out correctly.

hint:
factorise (a^3-b^3)
0
Jonny W
Badges: 8
Rep:
?
#6
Report 15 years ago
#6
Using x^3 - y^3 = (x - y)(x^2 + xy + y^2),

1
= (n + 1) - n
= cbrt(n + 1)^3 - cbrt(n)^3
= [cbrt(n + 1) - cbrt(n)] [cbrt(n + 1)^2 + cbrt(n + 1)cbrt(n) + cbrt(n)^2]
= [cbrt(n + 1) - cbrt(n)] [cbrt(n^2 + 2n + 1) + cbrt(n^2 + n) + cbrt(n^2)]

(sum from n = 1 to 100) 1 / [cbrt(n^2 + 2n + 1) + cbrt(n^2 + n) + cbrt(n^2)]
= (sum from n = 1 to 100) [cbrt(n + 1) - cbrt(n)]
= cbrt(101) - 1
0
john !!
Badges: 14
Rep:
?
#7
Report Thread starter 15 years ago
#7
yeah that's right. another method:

a^2 + ab + b^2 = (a^3 - b^3)/(a - b)
1/(a^2 + ab + b^2) = (a - b)/(a^3 - b^3)

so 1/[3rt{1} + 3rt{2} + 3rt{4}] = (3rt 2 - 3rt 1)
ect. onto 1/[3rt{10000} + 3rt{10100} + 3rt{10201}] = (3rt 101 - 3rt 100)

all others in between cancel to leave 3rt 101 - 1
0
X
new posts
Back
to top
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

What are you most likely to do if you don't get the grades you were expecting?

Go through Clearing (217)
38.68%
Take autumn exams (167)
29.77%
Look for a job (19)
3.39%
Consider an apprenticeship (22)
3.92%
Take a year out (102)
18.18%
Something else (let us know in the thread!) (34)
6.06%

Watched Threads

View All