7/3x18/5=
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\dfrac{7}{3}\) x \(\dfrac{18}{5}\) = \(\dfrac{126}{15}\) = \(\dfrac{42}{5}\)
\(\dfrac{7}{3}\times\dfrac{18}{5}=\dfrac{7\times6\times3}{3\times5}=\dfrac{42}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có
a=1+2+4+...........+211
=1+[1+1]+[1+1+2]+[1+1+2+3]+..................+[1+1+2+3+...........+20] =[1+......+1]+[1+1+.....+1]+[2+......+2]+..........+[19+19]+20
=21+[1*20+2*19+...............+19*2+20]
=1561
![](https://rs.olm.vn/images/avt/0.png?1311)
X:8x32+X:3x18-X:5x10=120
32X:8+18X:3-10X:5=120
4X+6X-2X =120
(4+6-2)X =120
8X =120
X =120:8
X =15
Chúc bn học tốt
![](https://rs.olm.vn/images/avt/0.png?1311)
Nếu `a/-7=b/5` thì:
`A.a/-7=b/5=`\(\dfrac{b-a}{-7-5}\)
`B.a/-7=b/5=`\(\dfrac{a-b}{-7=5}\)
`C.a/-7=b/5=`\(\dfrac{b-a}{5-7}\)
D.`a/-7=b/5=`\(\dfrac{b-a}{5+7}\)
`---`
Vì `a/-7=b/5`
Theo tính chất tỉ lệ thức `-> a/-7=b/5=`\(\dfrac{b-a}{5-\left(-7\right)}=\dfrac{b-a}{5+7}\)
`->` Đáp án `D.`
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Chọn câu C: \(\frac{5}{2};\frac{7}{4};\frac{4}{5};\frac{7}{10}\)
\(\dfrac{7}{3}\) x \(\dfrac{18}{5}\) = \(\dfrac{126}{15}\) = \(\dfrac{42}{5}\)