Question

vă dau coroane va rog frumos a,b,si d .​

Answer 1

Răspuns:

Explicație pas cu pas:

 

Answer 2

Avem progresii aritmetice și vom folosi formula:

$\it S_n=\dfrac{(a_1+a_n)\cdot n}{2}$

a)

$\it S_{10}=\dfrac{(2+20)\cdot10}{2}=\dfrac{22\cdot10}{2}=11\cdot10=110$

b)

$\it S_{11}=\dfrac{(11+121)\cdot11}{2}=\dfrac{132\cdot11}{2} =66\cdot11=726$

d)

$\it a_n=a_1+(n-1)r=850 \Rightarrow 5+(n-1)\cdot5=850 \Rightarrow 5(1+n-1)=850 \Rightarrow \\ \\ \Rightarrow 5n=850\bigg|_{:5} \Rightarrow n=170\\ \\ \\ s_{170}=\dfrac{(5+850)\cdot170}{2}=855\cdot85=72675$