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C2 arithmetic sequence help

The question is the sum of the first 30 terms of an arithmetic sequence with the first term 7 is equal to 450, find the thirtieth term. I have tyres subbing into all three formulas but I don't get it

Reply 1

username1432214

formula of sum = n/2(a+l)

where a = first term, l = last term aka 30th term in your case

(edited 8 years ago)

Reply 2

SunDun111

Original post by Bealzibub

formula of sum = n/2(a+l)

where a = first term, l = last term aka 30th term in your case

Dosent work?

Reply 3

samb1234

Original post by SunDun111

Dosent work?

Yes it does. 450 = 30/2 (7+L)

Posted from TSR Mobile

Reply 4

SunDun111

Original post by samb1234

Yes it does. 450 = 30/2 (7+L)

Posted from TSR Mobile

Oh yeah sorry my bad,

Reply 5

SunDun111

Original post by samb1234

Yes it does. 450 = 30/2 (7+L)

Posted from TSR Mobile

One more question, these are the only questions believe it or not I can't do, in an arthimitic sequence the ninth term is 7 and the twenty ninth is equal to twice the fifth term. Find a and d. Straight away I know it's simultaneous and my first equation is 7 = a + 8d but how do I get my second

Reply 6

username1432214

Original post by SunDun111

twenty ninth term = 2 * 5th term

fifth term = a + 4d

twenty ninth term = 2(a+4d)

2(a+4d) = a + 28d

Reply 7

SunDun111

Original post by Bealzibub

twenty ninth term = 2 * 5th term

fifth term = a + 4d

twenty ninth term = 2(a+4d)

2(a+4d) = a + 28d

Don't understand where does 28 come from urghh my bad not good with sequences big weakness..

Reply 8

davros

Original post by SunDun111

Don't understand where does 28 come from urghh my bad not good with sequences big weakness..

28 = 29 - 1

You were doing OK when you set up the first equation because you saw where the numbers came from. You do exactly the same thing when you write down an expression for the 29th term :smile: