
Mark M. answered 07/12/19
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Mathematics Teacher - NCLB Highly Qualified
S10 = (10/2)(2a1 + 9d)
a1 = a2 - d
a1 = 10 - d
2a1 = 20 - 2d
S10 = 5(20 - 2d + 9d)
120 = 5(20 + 7d)
24 = 20 + 7d
4 = 7d
4/7 = d
a1 = 10 - 4/7