Question 835036
 Find S[5] for the arithmetic sequence with 
a[3]=39 and a[5] =61
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a(3) = a + 2d = 39
a(5) = a + 4d = 61
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Subtract to solve for "d":
2d = 22
d = 11
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Solve for "a":
a + 2d = 39
a = 39-2*11
a = 17
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To find S(5)
S(n) = (n/2)(a+L)
S(5) = (5/2)(17+61) = 5(39) = 195
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Cheers,
Stan H.