Question

calculati suma : S = 1+4+7+....+100
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Answer 1

Răspuns:

S=1+4+7+.......+100

$S=\frac{a_{1}+a_{n}}{2}*n \\ r=3\\a_{n}=a_{1}+(n-1)*r\\100=1+3n-3=> 3n=102=> n 102:2=> n=34\\S=\frac{1+100}{2}*34=101*(34:2)=101*17=1717$

Explicație pas cu pas: