Series Question Fitzpatrick 2U (1 Viewer)

[ADrew]

Hey:

Exercise 11(b), Question 25, Fitzpatrick 2U:

The sum of the first six terms of an arithmetic sequence is -12 and the sum of the first fourteen terms is 196. Find:
a) the sum of n terms.
b) the smallest value of n if the sum is to exceed 250.


Thanks for any help i can get.

 

[hel00]

its just simultaneous eqns and a quadratic inequality lol and why are doing maths so late

 

[ADrew]

No I was just posting this now, not necessarily working questions.

 

[ADrew]

Please disregard this thread as I have figured out the question.

 

[genius2011]

if you cant do that forget even thinking about doing 4unit ( or even 3unit ) , just drop out now

 

[hscishard]

6/2 x (2a +(6-1)d) = -12
14/2 x (2a+(14-1)d) = 196

Simulatenously solve a and d. Then sub it in the n/2 x (2a+(n-1)d)

next you go n/2 x (2a+(n-1)d) >250
Then solve for n

 

[ADrew]

if you cant do that forget even thinking about doing 4unit ( or even 3unit ) , just drop out now

LOL, I know, but it was easy once I realised what to do.

6/2 x (2a +(6-1)d) = -12
14/2 x (2a+(14-1)d) = 196

Simulatenously solve a and d. Then sub it in the n/2 x (2a+(n-1)d)

next you go n/2 x (2a+(n-1)d) >250
Then solve for n

Thanks, I figured it out.

 

[ADrew]

if you cant do that forget even thinking about doing 4unit ( or even 3unit ) , just drop out now

Anyway, considering I have no issues with harder topics such as calculus, etc, and I already know alot of the four unit content, it's just series and sequences that's an issue for me.