(12 × 18) : 3 = 12 :?× 18 =?× 18 =?
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$\frac{4}{3} = \frac{{4 \times 6}}{{3 \times 6}} = \frac{{24}}{{18}}$
Vậy quy đồng mẫu số hai phân số $\frac{4}{3}$ và $\frac{{12}}{{18}}$, ta được các phân số $\frac{{24}}{{18}}$ và $\frac{{12}}{{18}}$
Vậy câu đúng là a; câu sai là b , c
\(\dfrac{7}{6}\) + \(\dfrac{5}{12}\) - \(\dfrac{1}{18}\) - 1
= \(\dfrac{42}{36}\) + \(\dfrac{15}{36}\) - \(\dfrac{2}{36}\) - \(\dfrac{36}{36}\)
= \(\dfrac{19}{36}\)
\(\dfrac{13}{6}\) + \(\dfrac{5}{18}+3-\dfrac{7}{12}\)
= \(\dfrac{78}{36}+\dfrac{10}{36}+\dfrac{108}{36}-\dfrac{21}{36}\)
= \(\dfrac{175}{36}\)
3 + \(\dfrac{11}{4}-\dfrac{1}{12}-\dfrac{3}{16}\)
= \(\dfrac{144}{48}+\dfrac{132}{48}-\dfrac{4}{48}-\dfrac{9}{48}\)
= \(\dfrac{263}{48}\)
\(\dfrac{2}{5}\) x \(\dfrac{7}{4}\) - \(\dfrac{2}{5}\) x \(\dfrac{3}{4}\)
= \(\dfrac{2}{5}\) x ( \(\dfrac{7}{4}\) - \(\dfrac{3}{4}\))
= \(\dfrac{2}{5}\) x \(\dfrac{4}{4}\)
= \(\dfrac{2}{5}\)
\(\frac{15-\frac{15}{7}-\frac{15}{12}-\frac{15}{98}}{18-\frac{18}{7}-\frac{18}{12}-\frac{18}{98}}\)= \(\frac{15-15\left(\frac{1}{7}+\frac{1}{12}+\frac{1}{98}\right)}{18-18\left(\frac{1}{7}+\frac{1}{12}+\frac{1}{98}\right)}\)=\(\frac{15\left(1-\frac{1}{7}-\frac{1}{12}-\frac{1}{98}\right)}{18\left(1-\frac{1}{7}-\frac{1}{12}-\frac{1}{98}\right)}\)= \(\frac{15}{18}\)=\(\frac{5}{6}\)
\(\frac{15-\frac{15}{7}-\frac{15}{12}-\frac{15}{98}}{18-\frac{18}{7}-\frac{18}{12}-\frac{18}{98}}=\frac{15\left(1+\frac{1}{7}+\frac{1}{12}+\frac{1}{98}\right)}{18\left(1+\frac{1}{7}+\frac{1}{12}+\frac{1}{98}\right)}=\frac{15}{18}=\frac{5}{6}\)
\(15-\frac{15}{7}-\frac{15}{12}-\frac{1}{98}\)
\(=15\left(1-\frac{1}{7}-\frac{1}{12}-\frac{1}{98}\right)\)
\(=15x\frac{449}{588}=\frac{2245}{196}\)
\(18-\frac{18}{7}-\frac{18}{12}-\frac{18}{98}\)
\(=18\left(1-\frac{1}{7}-\frac{1}{12}-\frac{1}{98}\right)\)
\(=18x\frac{449}{588}=\frac{1347}{98}\)
x - 36 : 18 = 12
x - 2 = 12
x= 14
(x - 36) : 18 = 12
(x - 36) = 216
x = 216 + 36
x = 352
461 + (x - 45) = 387
461 + x - 45 = 387
x = 387 - 461 + 45
x = -29
(x-36):18=12
x-36=12x18
x-36=216
x=216+36
x= 252
x-36:18=12
x-2=12
x=14
461+(x-45)=387
x-45= 387 - 461
x-45=-74
x=-74+45
x=-29
a: =8/5-2/3=24/15-10/15=14/15
12/20-6/18=3/5-1/3=9/15-5/15=4/15
b: 18/21-12/27=6/7-7/9=54/63-49/63=5/63
8/12-20/50=2/3-2/5=10/15-6/15=4/15
\(a,\dfrac{8}{5}-\dfrac{18}{27}=\dfrac{8}{5}-\dfrac{2}{3}=\dfrac{14}{15}\\ \dfrac{12}{20}-\dfrac{6}{18}=\dfrac{3}{5}-\dfrac{1}{3}=\dfrac{4}{15}\\ b,\dfrac{18}{21}-\dfrac{12}{27}=\dfrac{6}{7}-\dfrac{4}{9}=\dfrac{26}{63}\\ \dfrac{8}{12}-\dfrac{20}{50}=\dfrac{2}{3}-\dfrac{2}{5}=\dfrac{4}{15}\)
(12 x 18) : 3 = 12 : 3 x 18 = 4 x 18 = 72
HỌC TỐT !