2 7 : 4 5 = 2 7 × 5 4 = 10 28 = 5 14

5 3   x   4 5   =   5 x 4 3 x 5   =   20 15   =   4 3

\(\frac{36}{45}=\frac{4}{5};\frac{42}{45}=\frac{14}{15}\)

Có: \(\frac{4}{5}=\frac{12}{15}<\frac{14}{15}\Rightarrow\frac{36}{45}<\frac{42}{45}\)

36/45=12/15                                                  hoặc        36/45 < 42/45

42/45=14/15

Vậy 14/15 > 12/15

`2/5 + 9/15= 2/5 + 3/5=5/5=1`

`15/45 + 25/30= 1/3 + 5/6= 2/6+5/6=7/6`

`28/32 + 45/72=7/8+ 5/8=12/8=3/2`

`8/28 + 5/30= 2/7 + 1/6= 12/42 + 7/42= 19/42`

a: =2/5+3/5=1

b: =1/3+2/3=1

c: =7/8+5/8=12/8=3/2

d: =2/7+3/7=5/7

\(a,\dfrac{2}{5}+\dfrac{3}{5}=\dfrac{5}{5}=1\\ b,\dfrac{1}{3}+\dfrac{2}{3}=\dfrac{3}{3}=1\\ c,\dfrac{7}{8}+\dfrac{5}{8}=\dfrac{12}{8}=\dfrac{3}{2}\\ d,\dfrac{1}{7}+\dfrac{3}{7}=\dfrac{4}{7}\)

\(a,\\ =\dfrac{5}{4}-\dfrac{3}{4}=\dfrac{2}{4}=\dfrac{1}{2}\\ b,\\ =\dfrac{2}{3}-\dfrac{2}{5}=\dfrac{10-6}{15}=\dfrac{4}{15}\\ c,\\ =\dfrac{5}{6}-\dfrac{1}{4}=\dfrac{20-6}{24}=\dfrac{14}{24}=\dfrac{7}{12}\\ d,\\ =\dfrac{4}{3}-\dfrac{1}{4}=\dfrac{12-3}{12}=\dfrac{9}{12}=\dfrac{3}{4}\)

 cảm ơn chị ạ

mn ơi e vít nhầm là tính rồi rút gọn mn sửa lại là rút gọn rùi tính

Ta có: \(x^2-2xy-4z^2+y^2\)

\(=\left(x^2-2xy+y^2\right)-4z^2\)

\(=\left(x-y\right)^2-4z^2=\left(x-y-2z\right)\left(x-y+2z\right)\)

\(=\left[6-\left(-4\right)-2\cdot45\right]\left[6-\left(-4\right)+2\cdot45\right]=-80\cdot100=-8000\)

x² - 2xy + y² - 4z²

= (x - y)² - (2z)²

= (x - y - 2z) (x - y + 2z)

Thay x = 6 ; y = -4 và z = 45 vào biểu thức ta được:

[6 - (-4) - 2 . 45] [6 - (-4) + 2 . 45]

= -80 . 100

= -8000