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[tex]\huge\boxed{\boxed{\boxed{\boxed{\sf a_n=a_p+(n-p)\!\!\cdot\! r}}}}[/tex]
[tex]\large\boxed{\begin{array}{l}\sf a_3=12\\\sf a_5=22\\\sf a_{20}=?\\\sf a_5=a_3+2r\\\sf 22=12+2r\\\sf 2r=22-12\\\sf 2r=10\\\sf r=\dfrac{10}{2}\\\sf r=5\\\sf a_{20}=a_5+15r\\\sf a_{20}=22+15\cdot5\\\sf a_{20}=22+75\\\sf a_{20}=97\end{array}}[/tex]
