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The sum of the first 18 terms of an arithmetic series is 576. The common difference is -8. Determine the value of the first term, t1.

 Feb 15, 2021
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S = (n/2) × (2a + (n−1)d)     substituting in what is given

576 = (18/2)(2a + (18-1)(-8)

               9 (2a -136)

576 = 18a - 1224

a1 = 100  

 

100 + 10(-8) =  20   = a11

 Feb 15, 2021

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