Answers

2016-06-29T22:00:42+08:00
D = an - a1 / n - 1 

an = last term 
a1 = first term 


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2016-06-30T07:02:56+08:00
A5=A1+(n-1)d
2=-10+(5-1)d
2+10=4d
12=4d
12/4=4/4d
d=3
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