![\left \{ {{3x+7y=2} \atop {2x-5y=1}} \right. ; =\ \textgreater \ \left \{ {{3x=2-7y} \atop {2x=1+5y}} \right. ; =\ \textgreater \ \left \{ {{x=(2-7y):3} \atop {x=(1+5y):2}} \right. \\ \\](https://tex.z-dn.net/?f=+%5Cleft+%5C%7B+%7B%7B3x%2B7y%3D2%7D+%5Catop+%7B2x-5y%3D1%7D%7D+%5Cright.+%3B++%3D%5C+%5Ctextgreater+%5C++++%5Cleft+%5C%7B+%7B%7B3x%3D2-7y%7D+%5Catop+%7B2x%3D1%2B5y%7D%7D+%5Cright.+%3B+++%3D%5C+%5Ctextgreater+%5C++++%5Cleft+%5C%7B+%7B%7Bx%3D%282-7y%29%3A3%7D+%5Catop+%7Bx%3D%281%2B5y%29%3A2%7D%7D+%5Cright.++%5C%5C++%5C%5C+)
(2 - 7у) : 3 = (1 + 5у) : 2
2(2 - 7у) = 3(1 + 5у)
4 - 14у = 3 + 15у
15у + 14у = 4 - 3
29у = 1
у = 1/29
х = (1 + 5 * 1/29) : 2 = (34/29) : 2 = 17/29
Ответ: (17/29; 1/29).
Х = 3y
( x - 6 ) - 5 = y + 3
Решение
Х - 11 = y + 3
3y - 11 = y + 3
2y = 14
y = 7 ( мальчиков )
3 • 7 = 21 ( девочек )
7 + 21 = 28 ( человек ) всего
D = b²-4ac
D = 4²-4*7*13= 16 - 364= -348
D<0 ⇒ уравнение не имеет решений на области действительных чисел