(An),a3=25,a10=-3 найти a,d арифммтическая прогресия (An),a1=7,a3=11 найти a18 s18

[latex]a _{3} =25 \ a _{10} =-3 \ a _{3} =a _{1} +2d\a _{10} =a _{1} +9d\ \ a _{1} +2d=25 \ a _{1} +9d=-3/*(-1) \ \ a _{1} +2d=25 \ -a _{1} -9d=3\ -7d=28\d=-4 \ a _{1} -8=25\ a _{1} =25+8=33[/latex][latex]a _{1} =7 \ a _{3} =11 \ a _{3} =a _{1} +2d \ 11=7+2d\2d+7=11\2d=11-7\2d=4\d=2 \ a _{18} =a _{1} +17d=7+17*2=7+34=41 \ S _{18} = frac{2*a _{1} +17d}{2} = frac{14+34}{2} = frac{48}{2} =24[/latex]