Chapter 11 - Further Topics in Algebra - 11.2 Arithmetic Sequences and Series - 11.2 Exercises - Page 1025: 35

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$\left( a_{m}-a_{n}\right) =\left( m-n\right) d\Rightarrow a_{18}-a_{8}=\left( 18-8\right) d\Rightarrow -85-\left( -15\right) =10d\Rightarrow d=-7\Rightarrow a_{8}=a_{1}+7d\Rightarrow -15=a_{1}+7\times \left( -7\right) \Rightarrow a_{1}=49-15=34$