Brightstar
Badges: 0
Rep:
?
#1
Report Thread starter 13 years ago
#1
Okay I can do part a and part b 1 but not part b2 - its driving me mad and I can't understand why the markscheme has got what its got on it
This is the question
8. (a) Prove that the sum of the first n positive integers is given by
1/2 n(n + 1). (4)
(b) Hence, find the sum of
(i) the integers from 100 to 200 inclusive,
(ii) the integers between 300 to 600 inclusive which are divisible by 3. (5)

Answer to b 1 is 15150

on the mark scheme for part b 2
All they have given is 15150 X 3
why why ?
and how else could you get the answer - its driving me mad - just as I thought I was fine :C
It says hence - so don't you have to use the formula n/2(n+1)
1
reply
Glutamic Acid
Badges: 17
#2
Report 13 years ago
#2
b)i)One way of doing it is the (Sum of the first 200 natural numbers) - (Sum of the first 99 natural numbers).
b)ii) It can be written as 3(100+101+102....200). See why they've given it as 15150 x 3?
0
reply
snafle
Badges: 0
#3
Report 13 years ago
#3
Think about the numbers you have-

[100->200] 100, 101, 102, 103 etc
[300->600] 300, 303, 306, 309 etc

sneaky edit there Glutamic
0
reply
Doji
Badges: 15
Rep:
?
#4
Report 13 years ago
#4
100 - 200 both inclusive, means 101 numbers in there...

a= 100, l=200



101[100 + 200] \div  2



= 101(300) \div 2
0
reply
Doji
Badges: 15
Rep:
?
#5
Report 13 years ago
#5
2nd one...

300 - 600 (both are divisible by 3)

so, a = 300, l = 600

But how many terms are divisible by 3 between the range they gave?

300 ..303...306 ... (d=3)

300 + (n-1)3 = 600 <== We equate it to 600, as its the largest possible value in the range that is divisible by 3, and it will give the highest n value, i.e. the highest term that is divisible by 3, and thus the number of terms.

Now we rearrange:

3n - 3 = 600 - 300

3n - 3 = 300

3n = 303

n = 101 (101 terms are divisible by 3)

101[300 + 600]/2

= 45450
1
reply
Glutamic Acid
Badges: 17
#6
Report 13 years ago
#6
Doji, you're right, but the question says 'hence', implying it wants to you to use the formula for the sum of the first n natural numbers. Sometimes it says 'hence, or otherwise' though. I'm not entirely sure how it's marked, you may lose method marks for doing that.
0
reply
Doji
Badges: 15
Rep:
?
#7
Report 13 years ago
#7
Yeah i realised, she did say how else she could get it though ... you would probably lose 1 or 2 marks... although its solomon after all, i doubt edexcel will ask stuff like this, but can't be under prepared!
0
reply
snafle
Badges: 0
#8
Report 13 years ago
#8
Surely it's bad practice to give the answer away immediately, instead just give a hint? You learn far more that way.
0
reply
Doji
Badges: 15
Rep:
?
#9
Report 13 years ago
#9
(Original post by snafle)
Surely it's bad practice to give the answer away immediately, instead just give a hint? You learn far more that way.
well i stuggled over this question so thought ym explanation could help her, i checked the asnwer out, and the method helped me, not the answer
0
reply
HiyaLulu
Badges: 0
Rep:
?
#10
Report 4 years ago
#10
could you please post a link to the markcheme?
0
reply
Notnek
  • Study Helper
Badges: 20
Rep:
?
#11
Report 4 years ago
#11
(Original post by HiyaLulu)
could you please post a link to the markcheme?
This is a 9 year old thread.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
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 factors affect your mental health the most right now?

Anxiousness about lockdown easing (59)
4.63%
Uncertainty around my education (195)
15.29%
Uncertainty around my future career prospects (141)
11.06%
Lack of purpose or motivation (181)
14.2%
Lack of support system (eg. teachers, counsellors, delays in care) (57)
4.47%
Impact of lockdown on physical health (72)
5.65%
Loneliness (112)
8.78%
Financial worries (45)
3.53%
Concern about myself or my loves ones getting/having been ill (58)
4.55%
Exposure to negative news/social media (58)
4.55%
Lack of real life entertainment (68)
5.33%
Lack of confidence in making big life decisions (112)
8.78%
Worry about missed opportunities during the pandemic (117)
9.18%

Watched Threads

View All
Latest
My Feed