Вот накалякал. Разбирайся :)
<span>xy/(x+y) = 5
xz/(x+z) = 7
yz/(y+z) = 9</span>
<span>===========</span>
<span /><span>xy = 5x + 5y
xz = 7x + 7z
yz = 9y + 9z</span>
<span>===========</span>
<span /><span>x(y-5) = 5y
x = 5y/(y-5)</span>
<span>-----</span><span>
5yz/(y-5) = 35y/(y-5) + 7z
5yz = 35y + 7z * (y-5)
5yz = 35y + 7yz - 35z
2yz + 35y = 35z
y(2z + 35) = 35z
y = 35z/(2z + 35) = z/(2z/35 + 1)</span>
<span>------</span>
<span /><span>35z^2/(2z + 35) = 315z/(2z + 35) + 9z
35z^2 = 315z + 9z*(2z + 35)
35z^2 = 315z + 18z^2 + 315z
17z^2 = 630z
z=630/17</span>
<span>-------</span>
<span />y = 35*630/(2*630/17 + 35)/17 = 35*630/(1260 + 595) = 22050/1855 = 630 / 53
<span>x = 5*630/(630/53 - 5)/53 = 5*630/((630/53 - 5)*53) = 5*630/365 = 630/73</span>
А=5 в=-21 с=4
D=441-80=361
х=21-19/10=0.2
х=21+19/10=4
1) 6.7- 2.5 = 4.2
2) 4.2 / 2.4 = 1.75