# Help urgent maths

Help
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 formula
(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
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

I 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.
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
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

I 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)