等差数列题!记等差数列{An}的前n项和为Sn,若S1=4,S4=20,则该数列的公差d=?3Q!
4个回答
s1=a1=4
s4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4*a1+6d
=4*4+6d
=20
所以d=(20-4*4)/6=2/3
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