15. $\frac{8}{40}$ $+$ $\frac{-36}{45}$ $=$ $\frac{1}{5}$ $+$ $\frac{-4}{5}$
$=$ $\frac{1-4}{5}$ $=$$\frac{-3}{5}$
16. $\frac{-8}{18}$ +$\frac{-15}{27}$ $=$ $\frac{-4}{9}$ $+$ $\frac{-5}{9}$$=$ $\frac{-4-5}{9}$ $= -1$
17.$\frac{13}{30}$ $+$$\frac{-1}{5}$ $=$$\frac{13}{30}$ $+$ $\frac{-6}{30}$
$=$ $\frac{13-6}{30}$ =$\frac{7}{30}$
18. $\frac{2}{21}$ $+$$\frac{1}{28}$ $=$ $\frac{14}{84}$$+$ $\frac{3}{84}$$ =$ $\frac{11}{84}$
19. $5+$ $\frac{-3}{4}$$=$ $\frac{20}{4}$ $+$ $\frac{-3}{4}$$=$$\frac{17}{4}$
20. $\frac{18}{24}$ $+$ $\frac{35}{-10}$ $=$ $\frac{3}{4}$$+$ $\frac{7}{-2}$ $=$ $\frac{3}{4}$$+$$\frac{-14}{4}$ $=$ $\frac{-11}{4}$
bài 2
1. $x=$$\frac{-1}{2}$ $+$$\frac{3}{4}$$=$ $\frac{-2}{4}$ $+$$\frac{3}{4}$$=$$\frac{1}{4}$
2. $\frac{x}{5}$ $=$$\frac{5}{6}$$+$$\frac{-19}{30}$ $=$$\frac{25}{30}$$+$$\frac{-19}{30}$= $\frac{6}{30}$$=$$\frac{1}{5}$
vậy $x=1$
3.$\frac{x}{15}$= $\frac{3}{5}$ +$\frac{-2}{3}$= $\frac{9}{15}$ +$\frac{-10}{15}$= $\frac{-1}{15}$
vậy $x=-1$
4. $\frac{11}{8}$+$\frac{13}{6}$ =$\frac{85}{x}$= $\frac{33}{24}$ +$\frac{52}{24}$= $\frac{85}{24}$
vậy x=$\frac{47}{24}$
5.x-$\frac{-6}{15}$=$\frac{4}{27}$ => x=$\frac{4}{27}$+$\frac{-6}{15}$= $\frac{20}{135}$+$\frac{-54}{135}$=$\frac{-34}{135}$
vậy x=$\frac{-34}{135}$
chúc bạn học tốt !!
~chúc mừng năm mới~
xin câu trả lời hay nhất về cho nhóm !!!
