a₁=-12; d=12; a₇=a₁+(7-1)d=-12+6*12=60;
S₇=(a₁+a₇)*7/2=48*7/2=168;
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a₁=-12; d=12; a₇=a₁+(7-1)d=-12+6*12=60;
S₇=(a₁+a₇)*7/2=48*7/2=168;