Bài 21 trang 46 SGK Toán 8 tập 1

Thực hiện các phép tính sau:

a) \( \dfrac{3x-5}{7}+\dfrac{4x+5}{7}\);

b) \( \dfrac{5xy-4y}{2x^{2}y^{3}}+\dfrac{3xy+4y}{2x^{2}y^{3}}\)

c) \( \dfrac{x+1}{x-5}+\dfrac{x-18}{x-5}+\dfrac{x+2}{x-5}\).

a) \( \dfrac{3x-5}{7}+\dfrac{4x+5}{7} \)

\(=  \dfrac{3x-5+4x+5}{7}=\dfrac{7x}{7}=x\)

b) \( \dfrac{5xy-4y}{2x^{2}y^{3}}+\dfrac{3xy+4y}{2x^{2}y^{3}} \)

\(=  \dfrac{5xy-4y+3xy+4y}{2x^{2}y^{3}}\)

\(=\dfrac{8xy}{2x^{2}y^{3}}=\dfrac{4}{xy^{2}}\)

c) \( \dfrac{x+1}{x-5}+\dfrac{x-18}{x-5}+\dfrac{x+2}{x-5}\)

\(=  \dfrac{x+1+x-18+x+2}{x-5}\)

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