Actually all of it..plzz if u have time 
itz ok al noor. i ll. c on this forum u ll find many topics wer dey hv discussed this n given sum formulas but dey dnt go into my head. on da last minute i thnk dey r confusing. so i ll tell u my method.
1. wen u hv a sequence wer da 1st diff is da same eg. 3,5,7,9 den find da difference between dem n multiply it wid 'n' so da formula wud becum "2n" in this case as da diff is 2. den du 2n eg.in this case wen n=1, 2n=2x1=2 den find hw much u need 2 get 2 da first term. so in this case it wud b +1, 2 get 4rm 2 to 3(which is da 1st term). so da nth formula wud b 2n+1. den try this wid n=2 so 2x2+1= 5. which is ur second term. ryt....this is hw i du it itz very easy once u hv got it. but if u want formulas check sum other subject topics der hv been discussion on this.
ummm 4 da 2nd 1 wer u hv 2 find da nth term wen da 2nd diff is da same i hv my method 4 this 2 but again on this site der r formulas written 4 it. da 2d method is abit long but i ll help u. however u just tell me which method u find better. if u find da formulas more easy den go ahead wid dat but if u find my method ok den tell me, i ll try 2 explain da 2nd method. gud luck.
