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The sixteenth term of an arithmetic progression is 40. The fifty-fifth term of this arithmetic progression is 157. Find the eighty-first term of this arithmetic progression. 
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By the formula~$d=\dfrac{a\_{m}-a\_{n}}{m-n}\textbackslash{} \Rightarrow\textbackslash{} d=\dfrac{a\_{55}-a\_{16}}{55-16}=\dfrac{157-40}{39}=3.$  By the formula~$a\_{n}=a\_{m}+\left( n-m\right)d\textbackslash{} \textbackslash{} \textbackslash{} \Rightarrow\textbackslash{} \textbackslash{} a\_{81}=a\_{55}+\left( 81-55\right)\times3=235.$ 