Question

дано; a1=7 .d=4. n=13. найти a13= ?, S13=?

Answer 1

а13=а1+12d, а13=7+12*4=7+48=55

S13=(а1+а13)*13:2, S13=(7+55)*13:2=62:2*13=31*13=403

Answer 2

$a_{1}=7$; $d=4$; $n=13$

 

$a_{13} - ?$; $S_{13} - ?$

 

$a_{13}=a_{1}+12d=7+12cdot4=7+48=55$

 

$S_{13}=(a_{1}+a_{13})cdot13:2=(7+55)cdot13:2=</var><var>62:2cdot13=31cdot13=403$