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b: =>x-3-5x+15=7
=>-4x+12=7
=>-4x=-5
hay x=5/4
c: =>y(x-7)-2(x-7)=5
=>(x-7)(y-2)=5
\(\Leftrightarrow\left(x-7;y-2\right)\in\left\{\left(1;5\right);\left(5;1\right);\left(-1;-5\right);\left(-5;-1\right)\right\}\)
hay \(\left(x,y\right)\in\left\{\left(8;7\right);\left(12;3\right);\left(8;-3\right);\left(2;1\right)\right\}\)
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a) x= 10; y = 25
b) x + 2 y + 10 = 1 5 => ( x = 2).5 = ( y = 10).1=> 5.x + 10 = y + 10
=> 5.x = y mà y – 3.x = 2
Nên x = 1; y = 5
c) x = 20 ; y = 25
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\(\Leftrightarrow\left(2x+1,2y-5\right)\in\left\{\left(1;15\right);\left(15;1\right);\left(-1;-15\right);\left(-15;-1\right);\left(3;5\right);\left(5;3\right);\left(-3;-5\right);\left(-5;-3\right)\right\}\)
\(\Leftrightarrow\left(x,y\right)\in\left\{\left(0;10\right);\left(7;3\right);\left(-1;-5\right);\left(-8;2\right);\left(1;5\right);\left(2;4\right);\left(-2;0\right);\left(-3;1\right)\right\}\)
Vì ( 15 - 2x ) . ( 2y - 5 ) = 35
=> 15 - 2x thuộc Ư( 35 )
2y - 5 thuộc Ư ( 35 )
Mà Ư ( 35 ) = { 1; 5 ; 7 ; 35 }
+) 2y - 5 = 1 thì 15 - 2x = 35
2y = 1 + 5 2x = 35 + 15
2y = 6 2x = 50
y = 6 : 2 x = 50 : 2
y = 3 ( chọn ) x = 25 ( chọn )
+) 2y - 5 = 5 thì 15 - 2x = 7
2y = 5 + 5 2x = 7 + 15
2y = 10 2x = 22
y = 10 : 2 x = 22 : 2
y = 5 ( chọn ) x = 11
+) 2y - 5 = 7 thì 15 - 2x = 5
2y = 7 + 5 2x = 5 + 15
2y = 12 2x = 20
y = 12 : 2 x = 20 : 2
y = 6 ( chọn ) x = 10
+) 2y - 5 = 35 thì 15 - 2x = 1
2y = 35 + 5 2x = 1 + 15
2y = 40 2x = 16
y = 40 : 2 x = 16 : 2
y = 20 ( chọn ) x = 8
Vậy ( x; y ) = ( 25 ; 3 ) ; ( 11 ; 5 ) ; ( 10 ; 6 ) ; ( 8 ; 20 )