Mark M. answered 10/02/19
Mathematics Teacher - NCLB Highly Qualified
a9 = a1 + 8 d
a3 = a1 + 2d
a9 = 2a3
a1 + 8d = 2(a1+ 2d)
a1 + 8d = 2a1 + 4d
4d = a1
a15 = a1 + 14d
a15 = 4d + 14d
a15 = 18d
80 = 18d
You can determine the Sum of a9 to a15.
Nazreen H.
asked 10/01/19In an arithmetic sequence, the 9th term is twice the 3rd term and the 15th term is 80. Find the common difference and the sum of the terms from the 9th to the 15th inclusive.
Mark M. answered 10/02/19
Mathematics Teacher - NCLB Highly Qualified
a9 = a1 + 8 d
a3 = a1 + 2d
a9 = 2a3
a1 + 8d = 2(a1+ 2d)
a1 + 8d = 2a1 + 4d
4d = a1
a15 = a1 + 14d
a15 = 4d + 14d
a15 = 18d
80 = 18d
You can determine the Sum of a9 to a15.
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