Question

an=17+2n
s12-? s5-13-?

Answer 1

A1=19

A5=17+2*5=27

A12=17+2*12=41

s12=(a1+a12)12/2=360

S5-13=(a1+a5)5/2-13=102

Answer 2

$a_n=17+2n\\a_1=17+2\cdot1=19\\\ a_4=17+2\cdot4=25\\a_{12}=17+2\cdot12=41\\a_{13}=17+2\cdot13=43\\ \\ S_{12}=\frac{(a_1+a_{12})\cdot12}{2} =(19+41)\cdot6= 360\\\\ S_{4}=\frac{(a_1+a_{4})\cdot4}{2} =(19+25)\cdot2=44\cdot2=88\\ \\S_{13}=S_{12}+a_{13}=360+43=403\\ \\ S_{5-13}=S_{13}-S_4=403-88=315\\ \\ OTBET:S_{12}=360;S_{5-13}=315$