已知等差数列按的前n项和为sn,且s2=10,s5=55,则过点P(n,an),Q(n+2,an+2)的直线的斜率

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已知等差数列按的前n项和为sn,且s2=10,s5=55,则过点P(n,an),Q(n+2,an+2)的直线的斜率
已知等差数列按的前n项和为sn,且s2=10,s5=55,则过点P(n,an),Q(n+2,an+2)的直线的斜率

已知等差数列按的前n项和为sn,且s2=10,s5=55,则过点P(n,an),Q(n+2,an+2)的直线的斜率
S5-S2=a3+a4+a5=45
a3+a5=2a4
所以a4=15
所以a1+3d=15
S2=a1+a2=2*a1+d=10
所以a1=3,d=4
所以k=[a(n+2)-an]/[(n+2)-n]=2d/2=4

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