The sum of the first 12 terms of the arithmetic sequence tn=15-3n is?

The sum of the first 12 terms of the arithmetic sequence tn=15-3n is?
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tn=15-3nt1=15-3x1=12t2=15-3x2=9d=t2-t1/d=9-12=-3tn=t1+(n-1)xdtn=12+(12-1)x-312+11x-3=12+(-33)=-21=tnSn=n/2x(tn+t1)=S12=12/2 x(-21 +12)=6 x -9=-54
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