Найдите сумму первых пяти членов ар.прогрессии,если сумма третьего и седьмого членов...

$\left \{ {{a3 + a7 = 13} \atop {a4 * a6 = 42}} \right.$
$\left \{ {{a1 + 2d + a1 + 6d = 13} \atop {a4 * a6 = 42}} \right.$
$\left \{ {{2a1= 13 - 8d} \atop {a4 * a6 = 42}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {(a1+3d)(a1+5d) = 42}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {(6.5-4d+3d)(6.5-4d+5d) = 42}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {(6.5-d)(6.5+d) = 42}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {42.25 -d²= 42}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {-d²= -0.25}} \right.$
$\left \{ {{a1= 6.5 - 4d} \atop {d= 0.5}} \right.$
a1 = 4.5
s5 = 2a1 + 4d / 2 * 5
s5 = 27.5
ответ 27.5