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a1=a,S10=10a+10(10−1)d2=10a+45d,S5=5a+5(5−1)d2=5a+10d,S10=4S5,10a+45d=4(5a+10d,10a+45d=20a+40d,5d=10a,d=0,ad=510=12,即a1:d=12.故選:B.
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