设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?

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设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?设Sn是等差数列｛an｝的前n项和,已知a1=3,a

设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?
设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?

设Sn是等差数列｛an｝的前n项和,已知a1=3,a2=11,则S7等于?
a2=a1+d,则d=11-3=8
带入等差数列前N项和公式
Sn=na1+n(n-1)d/2
S7=189

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