1+5+9+13+17+``````+77=?快急,
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1+5+9+13+17+``````+77=?快急,
1+5+9+13+17+``````+77=?
快急,
1+5+9+13+17+``````+77=?快急,
等差数列,公差为4
和为首项加乘以项数除以2
(1+77)*19/2
(77-1)/4+1=20
(1+77)*20/2=780
S={(1+77)*[(77-1)/4+1]}/2
数列通项:An=4n-3
77为数列的第20项
利用等差数列前N项和公式:
Sn=20*A1+20*(20-1)*d/2
=20*1+20*19*4/2
=780
等差数列求和
首项加末项 乘以项数除以2
具体(77+1)×20/2 自己算吧
(1+77)20/2