A=(a-b)+(a+b+c)-(a-b-c)

=a-b+a+b+c-a+b+c

=(a+a-a)+(-b+b+b)+(c+c)

= a+b+c.2

= a+b+2c

B=(a-b)-(b-c)+(c-a)-(a-b-c)

=a-b-b+c+c-a-a+b+c

=(a-a-a)+(-b-b+b)+(c+c+c)

= (-a)+ (-b) +c.3

= (-a)+(-b)+3c

C=(-a+b+c)-(a-b+c)-(a+b-c)

= (-a)+b+c-a+b-c-a-b+c

=(-a-a-a)+(b+b-b)+(c-c+c)

= (-a.3) +b+c

\(3^4.3^n=3^7\)

\(3^n=3^7:3^4\)

\(3^n=3^3\)

\(=>n=3\)

x-15-19=31

x-15 =31+19

x-15 =50

x=50+15

x=65

Vậy x=65

\(\left(5^{19}:5^{17}-4\right):7\)

\(=\left(5^2-4\right):7\)

\(=\left(25-4\right):7\)

\(=21:7\)

\(=3\)

xin lỗi mk nhầm

\(\frac{20}{a}=\frac{4}{5}\)

\(=>a=25\)

\(\frac{20}{a}>\frac{4}{5}\)

=> a ϵ{24;23;22;....1}

Vì a bé nhất => a=1

\(\frac{20}{a}=\frac{4}{5}\)

=> a=25

Vì \(\frac{20}{a}>\frac{4}{5}\)

=> a=24

Để 43* chia hết cho 5

=> * ϵ { 0;5}

Vì 43* chia hết cho 3

=> *=5 ( vì 430 không chia hết cho 3)

Ta có 435