How to solve these ? Plz help ! 1)1)if a,b,c,d and f are in ap then e - c is equal to ? ( Board 2009 , 1 marks ) 2)If in any decreasing ap , sum of all its terms is equal to -36, the sum of all its terms, except the last term is zero and the difference of the 10th and the 6th term is equal to -16, then first term of the series is ? ( Board 2011 , 3 marks ) 3)A number 15 is divided into 3 parts which are in AP and the sum of their squares is 83 . What is the smallest part ?Also , if an ap consists of 31 terms , in which the 16th term is m,then find sum of all the terms of this ap. (Board 2011 , 2+2 marks )

How to solve these ? Plz help ! 

1)1)if a,b,c,d and f are in ap then e - c is equal to ? ( Board 2009 , 1 marks )

2)If in any decreasing ap , sum of all its terms is equal to -36, the sum of all its terms, except the last term is zero and the difference of the 10th and the 6th term is equal to -16, then first term of the series is ?            ( Board 2011 , 3 marks )

3)A number 15 is divided into 3 parts which are in AP and the sum of their squares is 83 . What is the smallest part ?Also , if an ap consists of 31 terms , in which the 16th term is m,then find sum of all the terms of this ap. (Board 2011 , 2+2 marks )