IGCSE/GCSE/O & A Level/IB/University Student Forum
Qualification => Subject Doubts => IB => Math => Topic started by: ~HyDrOgEn~ on December 20, 2010, 04:14:52 pm
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Show that 28,23,18,13 ... , is arithmetic , .Hence find Un and the sum of the first n terms in the simplest form ??
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Hey golden girl , it was really nice to meet you after The IGCSE has finished , i appreciate that you are helping , but i didn't get it , can you please re Explain
thanks :D
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Nice to see you around too =D
I sent my friend a sg hopefully you'll find the answer later or perhaps by tomorrow =]
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Show that 28,23,18,13 ... , is arithmetic , .Hence find Un and the sum of the first n terms in the simplest form ??
a+(n-1)d
28+(n-1)-5
31-5n
second part
sum of first n numbers
n/2(2a+(n-1)d)
n/2(2*28+(n-1)-5)
(-5n^2+61n)/ 2
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Show that 28,23,18,13 ... , is arithmetic , .Hence find Un and the sum of the first n terms in the simplest form ??
For the sequence to be an arithmetic progression it should fit in the equation Un = a + (n-1)d where a : First term, d : common difference and n : term number.
The sequence is arithmetic since the common difference is constant, i.e U1 - U2 = U3 - U4
So from the sequence we can not that the common difference is -5 -----> U2 - U1 = 23 - 28 = -5 or U4 - U3 = 13 - 18 = -5.
So now we just need to replace a = 28 and d = -5 in the formula ----> Un = 28 + (n - 1)(-5) ----> Un = 33 - 5n
Requiem made a little mistake there. ;)
Equation for the sum of n terms in an A.P = 0.5n(2a + (n - 1)d)
Again you just need to replace a = 28 and d = -5 ----> Sn = 0.5n(2(28) + (n-1)(-5))
Just simplify to get Sn = 0.5n(61 - 5n) or Sn = (61n - 5n2)/2
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It was only 33 instead of 31 :O.You wrote the whole answer
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It was only 33 instead of 31 :O.You wrote the whole answer
Sorry dude.
Indeed you did it well but I just wanted to help Hydrogen to understand and not just copy the answer.
Hope you don't mind. :-[
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Oh okay
No problem
:)
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Oh okay
No problem
:)
Thanks for your comprehension. :D
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Thanks for your comprehension. :D
Which chapter is this from, from CIE Mathematics books (Pure mathematics, Mechanics or Statistics)?
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Which chapter is this from, from CIE Mathematics books (Pure mathematics, Mechanics or Statistics)?
It's not cie mathematics , Its IB :)
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It's not cie mathematics , Its IB :)
I asked the chapter. ;D
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Which chapter is this from, from CIE Mathematics books (Pure mathematics, Mechanics or Statistics)?
Pure Maths 1
Chapter 8 ----> Sequences
@ Hydrogen : It may be IB but it's also found in CIE. ;)
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Pure Maths 1
Chapter 8 ----> Sequences
@ Hydrogen : It may be IB but it's also found in CIE. ;)
I see. 8)
Unfortunately we are not yet done with this chapter.
We are going to start this chapter soon, after completing whole S1. :D
S1 is my left hand game now ! ::)
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I see. 8)
Unfortunately we are not yet done with this chapter.
We are going to start this chapter soon, after completing whole S1. :D
S1 is my left hand game now ! ::)
Ooh......................I had completed the whole p1 and p3 before moving to S1 and M1. ;D
Nice.......keep up the good work. :)
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Ooh......................I had completed the whole p1 and p3 before moving to S1 and M1. ;D
Nice.......keep up the good work. :)
How is M1 ?
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it's 99% physics and 1% math =] and needs practice practice ..... so on
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it's 99% physics and 1% math =] and needs practice practice ..... so on
I will rule it. :D
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iA =D