Giải bài 98 trang 49 - Sách giáo khoa Toán 7 tập 1

Đề bài

Tìm y biết:

Hướng dẫn giải

a) \(-\dfrac{3}{5}.y =\dfrac{21}{10} \Leftrightarrow y = \dfrac{21}{10}:(-\dfrac{3}{5})\Leftrightarrow y = \dfrac{21}{10}.(-\dfrac{5}{3}) \Leftrightarrow y = -\dfrac{7}{3}\)

b) \(y :\dfrac{3}{8}= -1\dfrac{31}{33} \Leftrightarrow y :\dfrac{3}{8}= -\dfrac{64}{33} \Leftrightarrow y = (-\dfrac{64}{33}).\dfrac{3}{8} \Leftrightarrow y = -\dfrac{8}{11}\)

c) \(1\dfrac{2}{5}.y+\dfrac{3}{7}=-\dfrac{4}{5} \Leftrightarrow\dfrac{7}{5}.y+\dfrac{3}{7}=-\dfrac{4}{5} \Leftrightarrow \dfrac{7}{5}.y=-\dfrac{4}{5}-\dfrac{3}{7}\)

\(\dfrac{7}{5}.y=-\dfrac{43}{35} \Leftrightarrow y=(-\dfrac{43}{35}):\dfrac{7}{5} \Leftrightarrow y =-\dfrac{43}{49}\)

d) \(-\dfrac{11}{12}.y+0,25=\dfrac{5}{6} \Leftrightarrow -\dfrac{11}{12}y=\dfrac{5}{6}-0,25\)

\(-\dfrac{11}{12}.y=\dfrac{7}{12} \Leftrightarrow y= \dfrac{7}{12} : (-\dfrac{11}{12})\Leftrightarrow y =-\dfrac{7}{11}\)