[tex]a_{1} +3a_3=22\\a_1+3(a_1+2r)=22\\4a_1+6r=22\\2a_1+3r=11[/tex]
[tex]a_2+2a_5=39\\a_1+r+2(a_1+4r)=39\\a_1+r+2a_1+8r=39\\3a_1+9r=39\\a_1+3r=13[/tex[tex]\left \{ {{a_1+3r=13} \atop {2a_1+3r=11}} \right. \\3r=13-a_1\\2a_1+13-a_1=11\\a_1=-2\\3r=13-(-2)\\3r=15\\r=5\\\left \{ {{a=-2} \atop {r=5}} \right.[/tex]
[tex]a_n=-2+(n-1)5=-2+5n-5=5n-7[/tex]
Liczba wyrazów od 10 do 20 włącznie to 11
[tex]a_1_0=5*10-7=50-7=43\\a_2_0=5*20-7=100-7=93\\\\S_1_1=\frac{43+93}{2} *11=748[/tex]
