Пояснення:

[tex]\displaystyle\\\left \{ {{a_5+a_8=82} \atop {a_3+a_9=76}} \right. \ \ \ \ \ \ \left \{ {{a_1+4d+a_1+7d=82} \atop {a_1+2d+a_1+8d=76}} \right. \ \ \ \ \ \ -\left \{ {{2a_1+11d=82} \atop {2a_1+10d=76}} \right. \\\\d=6.\\\\2a_1+10*6=76\\\\2a_1+60=76\\\\2a_1=16\ |:2\\\\a_1=8.\\\\S_{20}=\frac{2*8+(20-1)*6}{2} *20=\frac{16+19*6}{2}*20=(16+114)*10=1300.[/tex]

Відповідь: S₂₀=1300.