Sn=a1+a2+a3=3 Sn=a2+a3+a5=11 a1-? d-?
Выразим каждый член прогрессии через a1 и d : a2=a1+d a3=a1+2d a5=a1+4d Теперь составим и решим систему уравнений :
![\left \{ {{a1+a1+d+a1+2d=3} \atop {a1+d+a1+2d+a1+4d=11}} \right. \ \textless \ =\ \textgreater \ \left \{ {{3a1+3d=3} \atop {3a1+7d=11}} \right. \ \textless \ =\ \textgreater \
\left \{ {{a1=-1} \atop {d=2}} \right.](https://tex.z-dn.net/?f=+%5Cleft+%5C%7B+%7B%7Ba1%2Ba1%2Bd%2Ba1%2B2d%3D3%7D+%5Catop+%7Ba1%2Bd%2Ba1%2B2d%2Ba1%2B4d%3D11%7D%7D+%5Cright.+%5C+%5Ctextless+%5C+%3D%5C+%5Ctextgreater+%5C+++%5Cleft+%5C%7B+%7B%7B3a1%2B3d%3D3%7D+%5Catop+%7B3a1%2B7d%3D11%7D%7D+%5Cright.++%5C+%5Ctextless+%5C+%3D%5C+%5Ctextgreater+%5C++%0A+%5Cleft+%5C%7B+%7B%7Ba1%3D-1%7D+%5Catop+%7Bd%3D2%7D%7D+%5Cright.+ "\left \{ {{a1+a1+d+a1+2d=3} \atop {a1+d+a1+2d+a1+4d=11}} \right. \ \textless \ =\ \textgreater \ \left \{ {{3a1+3d=3} \atop {3a1+7d=11}} \right. \ \textless \ =\ \textgreater \
\left \{ {{a1=-1} \atop {d=2}} \right.")