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According to the formula, first find the number of terms, and then find the sum of arithmetic sequence. (1) 7 + 10 + 13 + 16. + 100 (2) 16 + 21 + 26 +. + 206 Everybody, you're smart! Be sure! There won't be any chance on Friday! Use the tail of the item!
(1)a1=7,an=100,d=3
a1+(n-1)d=an
n=32
Sn=(a1+an)n/2=(7+100)32/2=1712
(2)a1=16,an=206,d=5
a1+(n-1)d=an
16+5(n-1)=206
n=39
Sn=(a1+an)n/2=(16+206)39/2=4329
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