Solve for a_21
a_{21}=82
Assign a_21
a_{21}≔82
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a_{21}=2+20\times 4
Subtract 1 from 21 to get 20.
a_{21}=2+80
Multiply 20 and 4 to get 80.
a_{21}=82
Add 2 and 80 to get 82.
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