№ 1. (8 4/5 - 13,8 : 3 5/6) · 12/13 = 4 целых 4/5 = 4,8.
1) 13,8 : 3 5/6 = 138/10 : 23/6 = 69/5 · 6/23 = (3·6)/(5·1) = 18/5 = 3 3/5
2) 8 4/5 - 3 3/5 = 5 1/5 = 26/5
3) 26/5 · 12/13 = (2·12)/(5·1) = 24/5 = 4 4/5 = 4,8
№ 2. 1 - 0,15 : (11/12 - 0,75) = 0,1.
1) 11/12 - 0,75 = 11/12 - 3/4 = 11/12 - 9/12 = 2/12 = 1/6
2) 0,15 : 1/6 = 3/20 · 6/1 = (3·3)/(10·1) = 9/10 = 0,9
3) 1 - 0,9 = 0,1
№ 3. 8,3 - (3 5/12 - 1 1/3) : 5/12 = 3,3.
1) 3 5/12 - 1 1/3 = 3 5/12 - 1 4/12 = 2 1/12 = 25/12
2) 25/12 : 5/12 = 25/12 · 12/5 = 25/5 = 5
3) 8,3 - 5 = 3,3
Вот накалякал. Разбирайся :)
xy/(x+y) = 5
xz/(x+z) = 7
yz/(y+z) = 9
xy = 5x + 5y
xz = 7x + 7z
yz = 9y + 9z
x(y-5) = 5y
x = 5y/(y-5)
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)
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
y = 35*630/(2*630/17 + 35)/17 = 35*630/(1260 + 595) = 22050/1855 = 630 / 53
x = 5*630/(630/53 - 5)/53 = 5*630/((630/53 - 5)*53) = 5*630/365 = 630/73
Формула разности квадратов.
Что не понятно-спрашивай.