Giải bài 27 trang 37 sách bài tập Toán 6 – Cánh Diều Tập 2 — Không quảng cáo

Đề bài

Tính các tổng sau (tính hợp lí nếu có thể):

a) \(\frac{7}{{ - 27}} + \frac{{ - 8}}{{27}}\)

b) \(\frac{6}{{13}} + \frac{{ - 17}}{{39}}\)

c) \(\frac{{ - 17}}{{13}} + \frac{{25}}{{101}} + \frac{4}{{13}}\)

d) \(\frac{{ - 13}}{7} + \frac{3}{5} + \frac{{ - 1}}{7}\)

e) \(\frac{{ - 5}}{9} + \frac{8}{{15}} + \frac{4}{{ - 9}} + \frac{7}{{15}}\)

Lời giải chi tiết

a) \(\frac{7}{{ - 27}} + \frac{{ - 8}}{{27}} = \frac{{ - 7}}{{27}} + \frac{{ - 8}}{{27}} = \frac{{( - 7) + ( - 8)}}{{27}} = \frac{{ - 15}}{{27}} =- \frac{5}{9}\)

b) \(\frac{6}{{13}} + \frac{{ - 17}}{{39}} = \frac{{18}}{{39}} + \frac{{ - 17}}{{39}} = \frac{{18 + ( - 17)}}{{39}} = \frac{1}{{39}}\)

c)

\(\begin{array}{l}\frac{{ - 17}}{{13}} + \frac{{25}}{{101}} + \frac{4}{{13}} = \left( {\frac{{ - 17}}{{13}} + \frac{4}{{13}}} \right) + \frac{{25}}{{101}} = \frac{{ - 17 + 4}}{{13}} + \frac{{25}}{{101}}\\ = \frac{{ - 13}}{{13}} + \frac{{25}}{{101}} =  - 1 + \frac{{25}}{{101}} = \frac{{ - 101}}{{101}} + \frac{{25}}{{101}} = \frac{{ - 101 + 25}}{{101}} = \frac{{ - 76}}{{101}}\end{array}\)

d)

\(\begin{array}{l}\frac{{ - 13}}{7} + \frac{3}{5} + \frac{{ - 1}}{7} = \left( {\frac{{ - 13}}{7} + \frac{{ - 1}}{7}} \right) + \frac{3}{5} = \frac{{ - 13 + ( - 1)}}{7} + \frac{3}{5}\\ = \frac{{ - 14}}{7} + \frac{3}{5} =  - 2 + \frac{3}{5} = \frac{{ - 10}}{5} + \frac{3}{5} = \frac{{ - 7}}{5}\end{array}\)

e)

\(\begin{array}{l}\frac{{ - 5}}{9} + \frac{8}{{15}} + \frac{4}{{ - 9}} + \frac{7}{{15}} = \left( {\frac{{ - 5}}{9} + \frac{4}{{ - 9}}} \right) + \left( {\frac{7}{{15}} + \frac{8}{{15}}} \right)\\ = \left( {\frac{{ - 5}}{9} + \frac{{ - 4}}{9}} \right) + \frac{{15}}{{15}} = \frac{{ - 9}}{9} + \frac{{15}}{{15}} =  - 1 + 1 = 0\end{array}\)