Tìm \(x\) biết: \(\,\frac{3}{4} – \left( {x + \frac{1}{2}} \right) = \frac{4}{5}\)

Tìm \(x\) biết: \(\,\frac{3}{4} – \left( {x + \frac{1}{2}} \right) = \frac{4}{5}\)

A. \(x=\frac{{ – 11}}{{20}}\)

B. \(x=\frac{{ – 13}}{{20}}\)

C. \(x=\frac{{ – 11}}{{19}}\)

D. \(x=\frac{{ – 21}}{{20}}\)

Hướng dẫn

Chọn đáp án là: A

Phương pháp giải:

Coi \(\left( {x + \frac{1}{2}} \right)\) là một ẩn, ta tìm \(\left( {x + \frac{1}{2}} \right)\) bằng cách lấy \(\frac{3}{4} – \frac{4}{5}\) , kết quả được bao nhiêu ta trừ đi \(\frac{1}{2}\) để tìm ra \(x\).

\(\begin{array}{l}\,\frac{3}{4} – \left( {x + \frac{1}{2}} \right) = \frac{4}{5}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {x + \frac{1}{2}} \right) = \frac{3}{4} – \frac{4}{5}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x + \frac{1}{2}\,\,\,\, = \frac{{15 – 16}}{{20}}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x + \frac{1}{2}\,\,\,\, = \frac{{ – 1}}{{20}}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{ – 1}}{{20}} – \frac{1}{2}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\,\frac{{ – 11}}{{20}}\end{array}\)

Chọn A