Bài 1 trang 15 toán lớp 7 tập 1 CTSTTínha)$\frac{2}{15}$ +$(\frac{-5}{24})$...

a) $\frac{2}{15}$ + $\left( \frac{-5}{24} \right)$ = $\frac{2 \times 24}{15 \times 24}$ + $\left( \frac{-5}{24} \right)$ = $\frac{48}{360}$ + $\left( \frac{-5}{24} \right)$ = $\frac{48 - 75}{360}$ = $\frac{-27}{360}$ = $\frac{-3}{40}$

b) $\frac{-5}{9}$ - $\left( -\frac{7}{27} \right)$ = $\frac{-15}{27}$ - $(-\frac{7}{27})$ = $\frac{-15 + 7}{27}$ = $\frac{-8}{27}$

c) $\left( -\frac{7}{12} \right)$ + 0,75 = $\left( -\frac{7}{12} \right)$ + $\frac{3}{4}$ = $\left( -\frac{7}{12} \right)$ + $\frac{9}{12}$ = $\frac{2}{12}$ = $\frac{1}{6}$

d) $\left( \frac{-5}{9} \right)$ - 1,25 = $\frac{-5}{9}$ - $\frac{5}{4}$ = $\frac{-20}{36}$ - $\frac{45}{36}$ = $\frac{-65}{36}$

e) 0,34 $\times$ $(-\frac{5}{17})$ = $\frac{34}{100}$ $\times$ $(-\frac{5}{17})$ = $\frac{-34 \times 5}{100 \times 17}$ = $\frac{-170}{1700}$ = $-\frac{1}{10}$

g) $\frac{4}{9}$ : $(-\frac{8}{15})$ = $\frac{4}{9}$ $\times$ $\frac{-15}{8}$ = $-\frac{5}{6}$

h) $(1\frac{2}{3})$ + $(2\frac{1}{2})$ = $\frac{5}{3}$ + $\frac{5}{2}$ = $\frac{10}{6}$ + $\frac{15}{6}$ = $\frac{25}{6}$

i) $\frac{2}{5}$ $\times$ $(-1,25)$ = $\frac{2}{5}$ $\times$ $(-\frac{5}{4})$ = $-\frac{1}{2}$

k) $(-\frac{3}{5})$ + $(\frac{15}{-7})$ $\times$ $3\frac{1}{9}$ = $-\frac{3}{5}$ + $\frac{15 \times 28}{-7 \times 9}$ = $-\frac{3}{5}$ + $-\frac{420}{63}$ = $-\frac{3}{5}$ + $-\frac{420}{63}$ = $-\frac{3}{5}$ + $-\frac{420}{63}$ = $- \frac{168 + 1050}{315}$ = $- \frac{1218}{315}$ = $-4$