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∵a1=2×13+k=23+k,a2=S2−S1=(2×19+k)−(2×13+k)=−49,a3=S3-S2=(2×127+k) ;−(2×19+k) ;=−427,∴(−49)2=(−427) ;×(23+k),∴k=-2.故答案:-2.
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