Solve for S_18
S_{18}=396
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S_{18}≔396
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S_{18}=\frac{18\times 44}{2}
Add -12 and 56 to get 44.
S_{18}=\frac{792}{2}
Multiply 18 and 44 to get 792.
S_{18}=396
Divide 792 by 2 to get 396.
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