Series and Sequences Help Needed! (1 Viewer)

[jjnfg]

can someone plz help me w/ these q's?

1. a series has the nth term given by Tn=n^3 -5. find the sum of 4 terms.

2. the sum of n terms of 214+206+198+... is 2760. evaluate n.

3. evaluate:
7
(sigma) 1/n
3
as a fraction.

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i kno how to do 1 & 3 the easy junior high school way where u just sub the numbers in and add them together but is there like a formula to do it properly? cos wat if u get a no. like 55 on the top?

also, i did no. 2 but i kept gettin the wrong answer...

thanx for ure help...

 

[...]

u sure this is 4u??

this is just plugging in numbers into the given formula...(or i'm wrong big time)

for the first one, work out first term by sub n=1 and last term by n=4..use that formulas(forgot which) and yea...

second, work out the difference or the pattern and sub in the numbers into formula, remembering that this time...Tn=2760 and u want n...

3, thats like the first one, sub n=3 and n=7 and then use n=3 as 'a' and n=7 as last term..use the sum up formula

 

[jjnfg]

yea but for 1, you have to find out whether it is an arithmetic or a geometric series first. so u find out T1, T2 & T3...then u see if T3-T2=T2-T1 or if T3/T2=T2/T1 but it doesnt fit into either...so its not a geo or an arith...unless IM doin something totally wrong...
same with question 3...

then for question 2...its sum of and you can work out that its a arith series with d=-8 and a=214. so u use the formula Sn=a+d(n-1)
>>2760=214-8(n-1)
=214-8n+8
8n=-2760+214+8
n=-317.25
and it cant be cos for one, n is negative and secondly, it isnt a whole number...

 

[ND]

For the 1st and 3rd, just do it the way you did it. (btw i assume it's supposed to say the first 4 terms)

For teh 2nd, you're using the wrong formula; you need the sum formula, not the term one.

 

[ftist]

aah...thanx for that so u use the Sn=n/2(2a+d(n-1))...alrite...thanx ppls