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For part c do I have to write all the terms out or can I use some kinda formula?? And for the rest as well is there a formulaIMG_2682.jpegIMG_2683.jpeg
(edited 8 months ago)
considering none of the sums go greater than 8 i'd assume you have to write all the terms out
from your working, though, it seems as though you've got it
Reply 2
Original post by AngryHomer
considering none of the sums go greater than 8 i'd assume you have to write all the terms out
from your working, though, it seems as though you've got it

But if it was a longer sequence what would I do… like if it was up to 50 😭 plz help also for this q

IMG_2686.jpegIMG_2687.jpegI got the wrong answer but idk why the black is the ms answer
(edited 8 months ago)
well because the sequence starts from 5 you can't just plug it into your formula - you need to do the (sum from 1 to 8) - (sum from 1 to 4) to get the overall sum

as for terms, it's the same as before, just plug in the numbers. i don't think they'd give it from n = 1 to 50 because that's going into further maths territory and they would have given you examples to practice on if that were the case, but identifying that it's a geometric series, as you did, is the key idea i would say.
Reply 4
Original post by AngryHomer
well because the sequence starts from 5 you can't just plug it into your formula - you need to do the (sum from 1 to 8) - (sum from 1 to 4) to get the overall sum

as for terms, it's the same as before, just plug in the numbers. i don't think they'd give it from n = 1 to 50 because that's going into further maths territory and they would have given you examples to practice on if that were the case, but identifying that it's a geometric series, as you did, is the key idea i would say.


R u sure my teacher said you can do the sequence from 5 to 8 by making the r value 4 because there’s 4 terms
Original post by Alevelhelp.1
R u sure my teacher said you can do the sequence from 5 to 8 by making the r value 4 because there’s 4 terms


okay well, just go with what your teacher says, they have a degree and i dont want to confuse you.
hope i was able to help
Reply 6
Original post by AngryHomer
okay well, just go with what your teacher says, they have a degree and i dont want to confuse you.
hope i was able to help

But I’m getting it wrong with her method
Original post by Alevelhelp.1

IMG_2687.jpegI got the wrong answer but idk why the black is the ms answer

It looks as though you have used the formula for the infinite sum (a / (1 - r)), but what you need is S4.
Original post by Alevelhelp.1
But if it was a longer sequence what would I do… like if it was up to 50.

If you're referring to the "sin(90r)" series, you can reason that the sum of the first four terms is 1 + 0 - 1 + 0 = 0. Likewise the sum of terms u5 to u8 is also 1 + 0 - 1 + 0 = 0. Then S50 will be 12 x 0 + 1 + 0 = 1.
(edited 8 months ago)

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