1 phút = 60 giây = 1/60 giờ

1 giây = 1/60 phút = 1/3600 giờ

a là chia het k phai ls;

Ta có số thứ hai gấp 5 lần số thứ nhất.

Theo sơ đồ, ta có hiệu số phần bằng nhau là:

5 - 1 = 4 (phần)

Số  thứ nhất là:

60 : 4 = 15

Số thứ hai là:

15 x 5 = 75 

Nha

Bài 1:

a) \(\dfrac{3x^2-5}{x^2-5x}+\dfrac{5-15x}{5x-25}\)

\(=\dfrac{3x^2-5}{x\left(x-5\right)}+\dfrac{5\left(1-3x\right)}{5\left(x-5\right)}\)

\(=\dfrac{3x^2-5}{x\left(x-5\right)}+\dfrac{1-3x}{x-5}\)

\(=\dfrac{3x^2-5}{x\left(x-5\right)}+\dfrac{x\left(1-3x\right)}{x\left(x-5\right)}\)

\(=\dfrac{3x^2-5+x\left(1-3x\right)}{x\left(x-5\right)}\)

\(=\dfrac{3x^2-5+x-3x^2}{x\left(x-5\right)}\)

\(=\dfrac{-5+x}{x\left(x-5\right)}\)

\(=\dfrac{x-5}{x\left(x-5\right)}\)

\(=\dfrac{1}{x}\)

b) \(\dfrac{4+x^3}{x-3}-\dfrac{2x+2x^2}{x-3}+\dfrac{2x-13}{x-3}\)

\(=\dfrac{\left(4+x^3\right)-\left(2x+2x^2\right)+\left(2x-13\right)}{x-3}\)

\(=\dfrac{4+x^3-2x-2x^2+2x-13}{x-3}\)

\(=\dfrac{x^3-2x^2-9}{x-3}\)

\(=\dfrac{x^3-3x^2+x^2-9}{x-3}\)

\(=\dfrac{x^2\left(x-3\right)+\left(x-3\right)\left(x+3\right)}{x-3}\)

\(=\dfrac{\left(x-3\right)\left(x^2+x+3\right)}{x-3}\)

\(=x^2+x+3\)

c) \(\dfrac{2}{x-5}+\dfrac{x-25}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{2\left(x+5\right)}{\left(x+5\right)\left(x-5\right)}+\dfrac{x-25}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{2\left(x+5\right)+x-25}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{2x+10+x-25}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{3x-15}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{3\left(x-5\right)}{\left(x+5\right)\left(x-5\right)}\)

\(=\dfrac{3}{x+5}\)

d) Đề sai?

Bài 2:

\(A=2\left(x+1\right)+\left(3x+2\right)\left(3x-2\right)-9x^2\)

\(A=2x+2+9x^2-4-9x^2\)

\(A=2x-2\)

\(A=2\left(x-1\right)\)

Thay x = 15 vào A ta được:

\(A=2\left(15-1\right)\)

\(A=2.14=28\)

a) \(\dfrac{x^2-4}{2x^2-4x}\)

\(=\dfrac{\left(x-2\right)\left(x+2\right)}{2x\left(x-2\right)}\)

\(=\dfrac{x+2}{2x}\)

b) \(\dfrac{2x-x^2}{x^2-4x+4}\)

\(=\dfrac{x\left(2-x\right)}{\left(x-2\right)^2}\)

\(=\dfrac{x\left(2-x\right)}{\left(2-x\right)^2}\)

\(=\dfrac{x}{2-x}\)

c) \(\dfrac{x^2-xy-x+y}{x^2+xy-x-y}\)

\(=\dfrac{x\left(x-y\right)-\left(x-y\right)}{x\left(x+y\right)-\left(x+y\right)}\)

\(=\dfrac{\left(x-y\right)\left(x-1\right)}{\left(x+y\right)\left(x-1\right)}\)

\(=\dfrac{x-y}{x+y}\)

d) \(\dfrac{5x^2+10x+5}{x+x^2}\)

\(\dfrac{5\left(x^2+2x+1\right)}{x\left(1+x\right)}\)

\(=\dfrac{5\left(x+1\right)^2}{x\left(x+1\right)}\)

\(=\dfrac{5\left(x+1\right)}{x}\)

e) \(\dfrac{3x^2+3x}{\left(x+1\right)\left(2x+6\right)}\)

\(=\dfrac{3x\left(x+1\right)}{\left(x+1\right).2\left(x+3\right)}\)

\(=\dfrac{3x}{2\left(x+3\right)}\)

f) \(\dfrac{\left(2-3x\right)\left(x+1\right)}{x^2+2x+1}\)

\(=\dfrac{\left(2-3x\right)\left(x+1\right)}{\left(x+1\right)^2}\)

\(=\dfrac{2-3x}{x+1}\)

63/4 = 18/1

Số cần tìm là :

100 : 5 = 20

Đ / S ; 20

Ai tích mình mình tích lại

mik nghĩ là 101

1,2 giờ 

để  \(\frac{n}{n+1}\)  là phân số tối giản thì   ƯCLN (n,n+1)= 1 

gọi ƯCLN (n,n+1) = d 

=> n chia hết cho d và n+1 chia hết cho d 

=>n+1 - n chia hết cho d 

=> 1 chia hết cho d 

mà d là ước chung lớn nhất (n,n+1) nên d = 1