(а-b)(a+b)=a²-b²...............................
Если х + у = 13, то у = 13 - х.
13 - х = - х² + 5х + 5
х² - 5х - 5 + 13 - х = 0
х² - 6х + 8 = 0
х₁ + х₂ = 6
х₁ * х₂ = 8
х₁ = 4; х₂ = 2
I. Если х = 4: y = 13 - 4 = 9
II. Если х = 2: y = 13 - 2 = 11
Ответ: (4; 9) и (2; 11)
1)x=0,(81)⇒100x=81,(81)
y=81,(81)⇒100y=8181,(81)
100y-y=99y=8181,(81)-81,(81)=8100
y=8100/99=900/11
100x=900/11
x=900/1100=9/11
2)x=0,(6)⇒10x=6,(6)
y=6,(6)⇒10y=66,(6)
10y-y=66,(6)-6,(6)=60
y=60/9=20/3
10x=20/3
x=20/30=2/3