\(a.\frac{19}{5}\cdot\frac{4}{7}+\frac{3}{7}\cdot\frac{19}{5}-\frac{4}{5}\)

\(=\frac{19}{5}\cdot\left(\frac{4}{7}+\frac{3}{7}\right)-\frac{4}{5}\)

\(=\frac{19}{5}\cdot1-\frac{4}{5}\)

\(=\frac{19}{5}-\frac{4}{5}=\frac{15}{5}=3\)

\(b.2\frac{2}{7}\cdot5\frac{2}{5}+\frac{16}{7}\cdot1\frac{3}{5}+\frac{1}{2}\)

\(=\frac{16}{7}\cdot\frac{27}{5}+\frac{16}{7}\cdot\frac{8}{5}+\frac{1}{2}\)

\(=\frac{16}{7}\cdot\left(\frac{27}{5}+\frac{8}{5}\right)+\frac{1}{2}\)

\(=\frac{16}{7}\cdot7+\frac{1}{2}\)

\(=16+\frac{1}{2}=\frac{33}{2}\)

\(c.\frac{3}{7}\cdot3\frac{3}{4}-\frac{3}{7}\cdot\frac{5}{4}-\frac{1}{4}\)

\(=\frac{3}{7}\cdot\frac{15}{4}-\frac{3}{7}\cdot\frac{5}{4}-\frac{1}{4}\)

\(=\frac{3}{7}\cdot\left(\frac{15}{4}-\frac{5}{4}\right)-\frac{1}{4}\)

\(=\frac{3}{7}\cdot\frac{5}{2}-\frac{1}{4}\)

\(=\frac{15}{14}-\frac{1}{4}=\frac{23}{28}\)

Chú ý: \(\cdot:\times\)

Ta có: A = 1.2.3 + 2.3.4 + 3.4.5 + ... + 18.19.20
=> 4A = 4(1.2.3 + 2.3.4 + 3.4.5 + ... + 18.19.20)
=> 4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 +...... +18.19.20.4
=> 4A = 1.2.3.4 + 2.3.4(5 - 1) + 3.4.5.(6 - 2) +..... +18.19.20.(21 - 17)
=> 4A = 1.2.3.4 + 2.3.4.5 - 2.3.4 + 3.4.5.6 - 2.3.4.5 + ..... + 17.18.19.20 - 17.18.19.20
=> 4A =18.19.20
=> 4A = 6840
=> A = 1710

Đặt A=1.2.3+2.3.4+3.4.5+4.5.6+...+18.19.20

4A=(1.2.3+2.3.4+3.4.5+4.5.6+...+18.19.20)4

4A=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+4.5.6(7-3)+...+18.19.20.(21-17)

4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+4.5.6.7-3.4.5.6+...+ 18.19.20.21-17.18.19.20
=>A =18.19.20.21

=>A+18.19.20.21/4

=> A=35910

a) x = \(\frac{-1}{7}\)và y =\(\frac{-5}{35}\)

vì \(\frac{-1}{7}=\frac{-1.5}{7.5}=\frac{-5}{35}\)

=> x = y 

b) \(x=\frac{5}{9}\)và \(y=\frac{1}{4}\)

x và y không bằng nhau

a,x=-1/7 và y=-5/35 

Bằng nhau vì 

\(\frac{-5}{35}=\frac{-1}{7}\)

b,x=5/19 và y=1/4 

không bằng nhau

k nha

a) x = −1/7 và y =−5/35 

vì −1/7 =−1.5/7.5 =−5/35 

=> x = y 

b) x=5/9 và y=1/4 

x và y không bằng nhau

1. TÍNH 

`5/7 xx 4 : 5/9 = 5/7 xx 4 xx 9/5 = 20/7 xx 9/5 = 36/7`

`4/9 : 2 xx 5/7 = 4/9 xx 1/2 xx 5/7 = 2/9 xx 10/63 `

`8 xx 2/3 : 1/2= 8xx 2/3 xx 2/1 = 8 xx 2/3 xx 2 = 16/3 xx 2=32/3`

Ta có : A= 5^1+5^2+...+5^20= (5+5^2)+(5^3+5^4)+...+(5^19+5^20)=5+5^2*(1+5+5^2+5^3+5^4+...+5^19)= 30*(1+5^2+5^3+5^4+...+5^19) chia hết cho 30 

Vậy A chia hết cho 30

\(25.5.4.125.20.8\)

\(=\left(25.4\right).\left(5.20\right).\left(125.8\right)\)

\(=100.100.1000\)

\(=10000000\)

25*(5+5)*4*20*8

=250*20+4+8

= 5000*12

=60000

đúng mk tích cho .

\(A=\left(1+3+5+......+19\right)-\left(2+4+6+..........+20\right)\)

\(=\left(\frac{\left(19+1\right).10}{2}\right)-\left(\frac{\left(20+2\right).10}{2}\right)\)

\(=100-110\)

\(=-10\)

vậy A ko chia hết cho cả 2,3,5 

\(Ư\left(A\right)=Ư\left(-10\right)=\left\{-10;-5;-2;-1;1;2;5;10\right\}\)