Câu 1 tính nhanh
a) $\frac{-5}{9}$ – $\frac{-5}{12}$
b)$\frac{-5}{12}$ : $\frac{15}{4}$
c) $\frac{1}{13}$. $\frac{8}{13}$ +$\frac{5}{13}$ .$\frac{1}{13}$ -$\frac{14}{13}$
d) A= $\frac{4}{1.5}$ +$\frac{4}{5.9}$+ $\frac{4}{9.13}$+…+ $\frac{4}{41.45}$
Câu 2 Tìm x biết
a) x=$\frac{1}{5}$ +$\frac{-3}{7}$
b) $\frac{3}{5}$- $\frac{4}{7}$:x= $\frac{-9}{10}$
c)x- ($\frac{-3}{4)}$ = $\frac{-2}{3}$ -$\frac{1}{2}$
Câu 1:
a) $\frac{-5}{9}$. $\frac{-5}{12}$ =$\frac{-5.(-5)}{9.12}$ =$\frac{25}{108}$
b) $\frac{-5}{12}$:$\frac{15}{4}$
$\frac{-5}{12}$ . $\frac{4}{15}$
= $\frac{-5.4}{12.15}$
= $\frac{-5.4}{4.3.(-5).(-3)}$
= $\frac{1}{3.(-3)}$
= $\frac{1}{-9}$
c)$\frac{1}{13}$ . $\frac{8}{13}$ + $\frac{5}{13}$ . $\frac{1}{13}$ – $\frac{14}{13}$
= $\frac{1.8}{13.13}$ + $\frac{5.1}{13.13}$ -$\frac{14}{13}$
= $\frac{1.8+5.1}{13.13}$ – $\frac{14}{13}$
= $\frac{13}{13.13}$ – $\frac{14}{13}$ = $\frac{1}{13}$ – $\frac{14}{13}$
= $\frac{-13}{13}$ = -1
d) A= $\frac{4}{1.5}$ + $\frac{4}{5.9}$ + $\frac{4}{9.13}$ +…+$\frac{4}{41.45}$
=$\frac{1}{1}$ – $\frac{1}{5}$ + $\frac{1}{5}$ – $\frac{1}{9}$ +$\frac{1}{9}$ – $\frac{1}{13}$ +…+$\frac{1}{41}$ – $\frac{1}{45}$
= 1- $\frac{1}{45}$
=$\frac{44}{45}$
Câu 2:
a)x=$\frac{1}{5}$ +$\frac{-3}{7}$
=>x = $\frac{7}{35}$ + $\frac{-15}{35}$
=>x=$\frac{-8}{35}$
b)$\frac{3}{5}$ – $\frac{4}{7}$ : x =$\frac{-9}{10}$
=> $\frac{4}{7}$ : x = $\frac{3}{5}$ – $\frac{-9}{10}$
=> $\frac{4}{7}$ : x = $\frac{6}{10}$ + $\frac{9}{10}$
=> $\frac{4}{7}$ : x = $\frac{3}{2}$
=> x = $\frac{3}{2}$ . $\frac{4}{7}$
=> x=$\frac{6}{7}$
c)x-($\frac{-3}{4}$)=$\frac{-2}{3}$ – $\frac{1}{2}$
=>x-($\frac{-3}{4}$)= $\frac{-4}{6}$ – $\frac{3}{6}$
=>x-($\frac{-3}{4}$)=$\frac{-7}{6}$
=> x=$\frac{-7}{6}$ + ($\frac{-3}{4}$)
=> x= $\frac{-14}{12}$ + $\frac{-9}{12}$
=> x=$\frac{-23}{12}$
Đáp án:
Giải thích các bước giải:
Câu 1
d)A=4/1.5+4/5.9+…..+4/41.45
A=1/1-1/5+1/5-1/9+…….+1/41-1/45
A=1/1-1/45
A= 44/45
b)-5/12:15/4=-1/9
a)-5/9 – -5/12=-5/36
Câu 2
a)x=22/35
b)3/5-4/7 ÷ x = -9/10
4/7 ÷ x = 3/5 – (-9/10)
4/7 ÷x =3/2
=>x= 8/12