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VT=(1/5+1/6+...+1/9)+(1/10+1/11+...+1/14)+(1/15+1/16+1/17)
VT<(1/5+...+1/5)+(1/10+...+1/10)+(1/15+1/15+1/15)=
=5/5+5/10+3/15=1+1/2+1/5<2
a)5.(-8).2.(-3)
=[5.(-2)].[(-8).(-3)]
=-10.24
=-240
b)(45-135+72)-(45+72)
=45-135+72-45-72
=(45-45)+(72-72)-135
=0+0-135
=-135
c)3.(-5)2.2+2.(-5)-20
=3.25.2+2(-5)-20
=75.2+(-10)-20
=150+(-30)
=120
d)86.(-46)+46.27-46.41
=86.(-1).46+46.27-46.41
=-86.46+46.27-46.41
=46(-86+27-41)
=46(-100)
-4600
e)34(15-10)-15(34-10)
=34.15-34.10-15.34+15.10
=(34.15-15.34)+(15.10-34.10)
=[34(15-15)]+[10(15-34)]
=34.0+10(-9)
=0+(-90)
=-90
\(\left(-\dfrac{7}{12}+\dfrac{5}{8}\right)-\left(-\dfrac{3}{8}+\dfrac{5}{12}\right)\)
\(=-\dfrac{7}{12}+\dfrac{5}{8}-\left(-\dfrac{3}{8}\right)-\dfrac{5}{12}\)
\(=\left(-\dfrac{7}{12}-\dfrac{5}{12}\right)+\left(\dfrac{5}{8}+\dfrac{3}{8}\right)\)
\(=-1+1=0\)
\(\left(-\dfrac{7}{12}+\dfrac{5}{8}\right)-\left(-\dfrac{3}{8}+\dfrac{5}{12}\right)\)
\(=-\dfrac{7}{12}+\dfrac{5}{8}+\dfrac{3}{8}-\dfrac{5}{12}\)
\(=\left(-\dfrac{7}{12}-\dfrac{5}{12}\right)+\left(\dfrac{5}{8}+\dfrac{3}{8}\right)\)
\(=-\dfrac{12}{12}+\dfrac{8}{8}\)
\(=-1+1\)
\(=0\)
a) (-2) . ( x+7 ) + (-5) = 7
<=>(-2).(x+7)=7+5
<=>x+7=12:(-2)
<=>x+7=-6
<=>x=(-6)-7
<=>x=-13
Vậy x=-13
b)(x+4) : (-7) = 14
<=>x+4=14 x (-7)
<=>x+4=-98
<=>x=-98-4
<=>x=-102
Vậy x= -102
c) 72 : ( x+5) - 4 = -12
<=>72:(x+5)=(-12)+4
<=>x+5=72:(-8)
<=>x+5=-9
<=>x=-9-5
<=>x=-14
Vậy x= -14
d) (x+3) : (-6 ) + 12 = 8
<=>(x+3) :(-6)=8-12
<=>x+3=(-4)x(-6)
<=>x+3=24
<=>x=24-3
<=>x=21
Vậy x= 21
`-5/8xx5/12+5/8xx(-7/12)+17/8`
`=5/8xx(-5/12)+5/8xx(-7/12)+17/8`
`=5/8xx(-5/12-7/12)+17/8`
`=5/8xx(-12)/12+17/8`
`=5/8xx(-1)+17/8`
`=-5/8+17/8`
`=12/8=3/2`
\(-\dfrac{5}{8}\cdot\dfrac{5}{12}+\dfrac{5}{8}\cdot\left(-\dfrac{7}{12}\right)+\dfrac{17}{8}\)
\(=-\dfrac{5}{8}\cdot\dfrac{5}{12}+-\dfrac{5}{8}\cdot\dfrac{7}{12}+\dfrac{17}{8}\)
\(=-\dfrac{5}{8}\left(\dfrac{5}{12}+\dfrac{7}{12}\right)+\dfrac{17}{8}\)
\(=-\dfrac{5}{8}\cdot1+\dfrac{17}{8}\)
\(=-\dfrac{5}{8}+\dfrac{17}{8}\)
\(=\dfrac{12}{8}=\dfrac{3}{2}\)
9:
a: Gọi ƯCLN(4n+8;2n+3)=d
=>4n+8-4n-6 chia hết cho d
=>2 chia hết cho d
mà 2n+3 lẻ
nên d=1
=>PSTG
b: Gọi d=ƯCLN(4n+5;5n+6)
=>20n+25-20n-24 chia hết cho d
=>1 chia hêt cho d
=>d=1
=>PSTG
44
thanks