sory lớp 8 ko bt

=\(\frac{30x^2y^3}{8x^2y^2}\)

Ta có :

\(\frac{6x^2y^2}{8xy^5}=\frac{3x}{4y^3}\)

\(\frac{x^2-xy}{5xy-5y^2}=\frac{x\left(x-y\right)}{5y\left(x-y\right)}=\frac{x}{5y}\)

Hok tốt !

8dm2 = 800 cm2

8 dm² = 800 cm²

mk chắc chắn 100% là 99m<9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999

Gọi d là ước của 2 số: 3a+7&4a+9

+)3a+7\(⋮\)d

=>4.(3a+7)\(⋮\)d

=>12a+28\(⋮\)d(1)

+)4a+9\(⋮\)d

=>3.(4a+9)\(⋮\)d

=>12a+27\(⋮\)d(2)

Từ (1) và(2) suy ra(12a+28)-(12a+27)\(⋮\)d

                      =>12a+28-12a-27\(⋮\)d

                        =>1\(⋮\)d

                       =>d=1

Vì d=1 nên 3a+7&4a+9 là 2 số ntố cùng nhau

Chúc bn học tốt

a) \(A=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{n\left(n+1\right)}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{n}-\frac{1}{n+1}=1-\frac{1}{n+1}\)

b) \(B=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{n\left(n+1\right)\left(n+2\right)}=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{n\left(n+1\right)\left(n+2\right)}\right)\)

         \(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{n\left(n+1\right)}-\frac{1}{\left(n+1\right)\left(n+2\right)}\right)=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{\left(n+1\right)\left(n+2\right)}\right)\)

\(=\frac{1}{4}-\frac{1}{2\left(n+1\right)\left(n+2\right)}\)