21y³+7y²-45y-15=7y²(3y+1)-15(3y+1)=(3y+1)(7y²-15)
![\left \{ {{ \frac{x}{5}*30+ \frac{y}{6}*30=- \frac{9}{10}*30 } \atop { \frac{x}{8}*56- \frac{y}{7} *56=- \frac{73}{56} *56}} \right.](https://tex.z-dn.net/?f=+%5Cleft+%5C%7B+%7B%7B+%5Cfrac%7Bx%7D%7B5%7D%2A30%2B+%5Cfrac%7By%7D%7B6%7D%2A30%3D-+%5Cfrac%7B9%7D%7B10%7D%2A30+++%7D+%5Catop+%7B+%5Cfrac%7Bx%7D%7B8%7D%2A56-+%5Cfrac%7By%7D%7B7%7D++%2A56%3D-+%5Cfrac%7B73%7D%7B56%7D+%2A56%7D%7D+%5Cright.+)
![\left \{ {{6x+5y=-27} \atop {7x-8y=-73}} \right.](https://tex.z-dn.net/?f=+%5Cleft+%5C%7B+%7B%7B6x%2B5y%3D-27%7D+%5Catop+%7B7x-8y%3D-73%7D%7D+%5Cright.+)
Умножим первое уравнение на 8, а второе на 5, получим:
![+\left \{ {{48x + 40y = - 216} \atop {35x - 40y = -365}} \right.](https://tex.z-dn.net/?f=+%2B%5Cleft+%5C%7B+%7B%7B48x+%2B+40y+%3D+-+216%7D+%5Catop+%7B35x+-+40y+%3D+-365%7D%7D+%5Cright.+)
________________
83x = - 581
x = - 7
6 * 7 + 5y = - 27
42 + 5y = - 27
5y = - 27 - 42
5y = - 69
y = - 13,8
Ответ : (7 ; - 13,8)
Смотрите решение в прикреплённом файле.
(0,36-0,16)-(1,5н-1,5р)=0,20