Помогите пожалуйста с номерами 17, 18, 21

17. [latex] \frac{4-c^2}{5c-10}= \frac{(2-c)(2+c)}{5(c-2)} =- \frac{c+2}{5} [/latex] 18. [latex] \left \{ {{x+y=9} \atop {x^2+y^2=41}} \right. [/latex] [latex] \left \{ {{(x+y)^2=81} \atop {x^2+y^2=41}} \right. [/latex] [latex] \left \{ {{x^2+2xy+y^2=81} \atop {x^2+y^2=41}} \right. [/latex] [latex] \left \{ {{2xy=40} \atop {x+y=9}} \right. [/latex] [latex] \left \{ {{xy=20} \atop {x+y=9}} \right. [/latex] По теореме Виета x и y - это корни квадратного уравнения t^2 - 9t + 20 = 0 (t - 4)(t - 5) = 0 x1 = 4; y1 = 5 x2 = 5; y2 = 4