Question

[👾thuii]

Giup mình với ah.

1- Tính :

A= 5. | x- 5 | - 3x + 1

2 - Tìm các số nguyên x,y ; sao cho :

a) 5/x - y/3 = 1/6                        b) 5/x + y/4 = 1/8

3- Tìm giá trị lớn nhất của Q = 27-2x/12-x ( x là số nguyên)

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Answer 1

[Nguyễn Đức Trí]

1) \(A=5.\left|x-5\right|-3x+1\)

\(A=\left[{}\begin{matrix}5.\left(x-5\right)-3x+1\left(x-5\ge0\right)\\5.\left(5-x\right)-3x+1\left(x-5< 0\right)\end{matrix}\right.\)

\(A=\left[{}\begin{matrix}5x-25-3x+1\left(x\ge5\right)\\25-5x-3x+1\left(x< 5\right)\end{matrix}\right.\)

\(A=\left[{}\begin{matrix}2x-24\left(x\ge5\right)\\26-8x\left(x< 5\right)\end{matrix}\right.\)

Answer 2

[Nguyễn Lê Phước Thịnh]

3:

\(Q=\dfrac{27-2x}{12-x}=\dfrac{2x-27}{x-12}\)

\(\Leftrightarrow Q=\dfrac{2x-24-3}{x-12}=2-\dfrac{3}{x-12}\)

Để Q lớn nhất thì \(2-\dfrac{3}{x-12}\) lớn nhất

=>\(\dfrac{3}{x-12}\) nhỏ nhất

=>x-12 là số nguyên âm lớn nhất

=>x-12=-1

=>x=11

Vậy: \(Q_{min}=2-\dfrac{3}{11-12}=2+3=5\) khi x=11

Bài 2:

a: \(\dfrac{5}{x}-\dfrac{y}{3}=\dfrac{1}{6}\)

=>\(\dfrac{15-xy}{3x}=\dfrac{1}{6}\)

=>\(15-xy=\dfrac{x}{2}\)

=>\(30-2xy=x\)

=>x+2xy=30

=>x(2y+1)=30

mà x,y nguyên

nên \(\left(x;2y+1\right)\in\left\{\left(30;1\right);\left(-30;-1\right);\left(2;15\right);\left(-2;-15\right);\left(10;3\right);\left(-10;-3\right)\right\}\)

=>\(\left(x,y\right)\in\left\{\left(30;0\right);\left(-30;-1\right);\left(2;7\right);\left(-2;-8\right);\left(10;1\right);\left(-10;-2\right)\right\}\)

b: \(\dfrac{5}{x}+\dfrac{y}{4}=\dfrac{1}{8}\)

=>\(\dfrac{20+xy}{4x}=\dfrac{1}{8}\)

=>\(\dfrac{40+2xy}{8x}=\dfrac{x}{8x}\)

=>40+2xy=x

=>x-2xy=40

=>x(1-2y)=40

mà x,y nguyên

nên \(\left(x;1-2y\right)\in\left\{\left(40;1\right);\left(-40;-1\right);\left(8;5\right);\left(-8;-5\right)\right\}\)

=>\(\left(x,y\right)\in\left\{\left(40;0\right);\left(-40;1\right);\left(8;-2\right);\left(-8;3\right)\right\}\)