Consider the 8th term of an arithmetic sequence as first equation;
\(\begin{array}{c}{a_1} + \left( {8 - 1} \right)d = 14\\{a_1} + 7d = 14\end{array}\)
Similarly, consider the 12th term of an arithmetic sequence as second equation;
\(\begin{array}{c}{a_1} + \left( {12 - 1} \right)d = 26\\{a_1} + 11d = 26\end{array}\)
Solve the two equations as follows:
\(\begin{array}{c}{a_1} + 11d = 26\\{a_1} + 7d = 14\\0 + 4d = 12\\d = 3\end{array}\)
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math