Question 195095
a(n) =a(1) + (n-1) d,,,,where  a(n) =  nth entry,,n=no entries,,d = difference  between  entries
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a(15) = 20 + (15-1) 3 = 20 + 42 = 62
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recursive,  a(n) =  a (n-1) +d
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sum = n(a(1) +a(n) ) /2= 15 *( 20 +62)/2 = 615
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