2/6 x 7 + 2/7 x 8 +...+ 2/70 x 71
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3 tháng 8 2016
a, (X+7)x8-14=66
=>(X+7)x8=66+14
=>(X+7)x8=80
=>X+7=80:8
=>X+7=10
=>X=10-7
=>X=3
b, 1+2+3+...+X=1711
<=>(X+1)xX:2=1711
=>(X+1)xX=1711x2
=>(X+1)xX=3422=59x58
Vì x<x+1 và (x+1)-x=1
=>x=58
c, X+3/5+Xx2/7=31/70
<=>Xx(1+2/7)+3/5=31/70
=>Xx9/7=31/70-3/5
=>Xx9/7=-11/70
=>X=-11/70:9/7
=>X=-11/90
d, 1/7+6/7xX=1/7
=>6/7xX=1/7-1/7
=>6/7xX=0
=>X=0:6/7
=>X=0
Chúc bạn học giỏi nha!!!
K cho mik với nhé cung bảo bình
CM
23 tháng 5 2017
a) 7 x 5 = 35 7 x 6 = 42 7 x 8 = 56 7 x 4 = 28
35 : 7 = 5 42 : 7 = 6 56 : 7 = 8 28 : 7 = 4
b) 42 : 7 = 6 35 : 7 = 5 63 : 7 = 9 16 : 2 = 8
14 : 7 = 2 49 : 7 = 7 7 : 7 = 1 48 : 6 = 8
28 : 7 = 4 70 : =10 21 : 7 = 3 56 : 7 = 8
TT
3
MT
13 tháng 10 2018
\(\frac{39}{40}-\frac{7}{9}\div\frac{2}{3}\left(\times\frac{3}{2}\right)\times\frac{2}{7}\)\(=\frac{39}{40}-\frac{7\times3\times2}{3\times3\times2\times7}=\frac{39}{40}-\frac{1}{3}\)\(=\frac{117}{120}-\frac{40}{120}=\frac{77}{120}\)
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\(\frac{20}{63}\div\frac{8}{9}\left(\times\frac{9}{8}\right)+\frac{5}{6}\times\frac{51}{70}=\frac{5\times4\times9}{9\times7\times4\times2}+\frac{5\times17\times3}{3\times2\times5\times14}\)\(=\frac{5}{14}+\frac{17}{28}=\frac{10+17}{28}=\frac{28}{28}=1\)
25 tháng 10 2021
\(1,\Leftrightarrow7x=42\Leftrightarrow x=6\\ 2,\Leftrightarrow36-4x=4\Leftrightarrow4x=32\Leftrightarrow x=8\\ 3,\Leftrightarrow3x=35\Leftrightarrow x=\dfrac{35}{3}\\ 4,\Leftrightarrow x-12=144\Leftrightarrow x=156\\ 5,\Leftrightarrow x-14=16\Leftrightarrow x=30\\ 6,\Leftrightarrow3x-24=\dfrac{148}{73}\Leftrightarrow3x=\dfrac{1900}{73}\Leftrightarrow x=\dfrac{1900}{219}\\ 7,\Leftrightarrow33+x=45\Leftrightarrow x=12\\ 8,Sai.đề\\ 9,\Leftrightarrow\left(x+9\right):2=39\Leftrightarrow x+9=78\Leftrightarrow x=69\\ 11,\Leftrightarrow2\left(x+7\right)=38\Leftrightarrow x+7=19\Leftrightarrow x=12\\ 13,\Leftrightarrow2\left(x-51\right)=66\Leftrightarrow x-51=33\Leftrightarrow x=84\)
25 tháng 10 2021
\(15,\Leftrightarrow x-19=9\Leftrightarrow x=28\\ 17,\Leftrightarrow\left(x-3\right):2=48\Leftrightarrow x-3=96\Leftrightarrow x=99\\ 19,\Leftrightarrow0x=46\Leftrightarrow x\in\varnothing\\ 8,Sai.đề\\ 14,\Leftrightarrow2x=209\Leftrightarrow x=\dfrac{209}{2}\\ 16,\Leftrightarrow2x+6=157\Leftrightarrow2x=151\Leftrightarrow x=\dfrac{151}{2}\\ 18,\Leftrightarrow5\left(x+4\right)=100\Leftrightarrow x+4=20\Leftrightarrow x=16\\ 20,\Leftrightarrow\left(3x-5\right)^3=27=3^3\Leftrightarrow3x-5=3\Leftrightarrow x=\dfrac{8}{3}\)
2 tháng 4 2020
Đặt B=719+718+717+...+712+71
71B=7110+719+718+717+...+712
71B-B=7110-71
70B=7110-71=>B=\(\frac{71^{10}-71}{70}\)
Ta có A=70.\(\frac{71^{10}-71}{70}\)
=7110-71
2 tháng 12 2022
a: =>33+x=71-36=35
=>x=2
b: =>(2x+6)=200-64=136
=>2x=130
=>x=65
c: =>x-19=76
=>x=95
e: =>x+9=39
=>x=30
f: =>x-8=20
=>x=28
g: =>2x=5*9+49=45+49=94
=>x=47
14 tháng 5 2018
b)\(N=\dfrac{yz}{x^2}+\dfrac{zx}{y^2}+\dfrac{xy}{z^2}\)
\(N=\dfrac{xyz}{x^3}+\dfrac{xyz}{y^3}+\dfrac{xyz}{z^3}\)
\(N=xyz\left(\dfrac{1}{x^3}+\dfrac{1}{y^3}+\dfrac{1}{z^3}\right)\)
Ta cm đẳng thức sau:\(x^3+y^3+z^3=3xyz\Leftrightarrow x+y+z=0\)
ĐT\(\Leftrightarrow x^3+y^3-3xyz=-z^3\)
\(\Leftrightarrow\left(x+y\right)\left(x^2-xy+y^2\right)-3xy=-z^3\)
\(\Leftrightarrow-zx^2+xyz-zy^2-3xyz=-z^3\)
\(\Leftrightarrow x^2+2xy+y^2=z^2\)
\(\Leftrightarrow\left(x+y\right)^2=z^2\)
\(\Leftrightarrow\left(-z\right)^2=z^2\)(luôn đúng)
Áp dụng\(\Rightarrow N=xyz.\dfrac{3}{xyz}=3\)
14 tháng 5 2018
a, (M-1)/70-71=m
m=(71^9+71^8....71+1)
71m=71^10+...71^2+71
70m=71^10-1
(M-1)/70=71^10+70
M-1=70(71^10+70)
M=70(71^10+70)-1
3 tháng 7 2018
1) \(A=\frac{7}{10\times11}+\frac{7}{11\times12}+\frac{7}{12\times13}+...+\frac{7}{69\times70}\)
\(A=7\times\left(\frac{1}{10\times11}+\frac{1}{11\times12}+\frac{1}{12\times13}+...+\frac{1}{69\times70}\right)\)
\(A=7\times\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(A=7\times\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(A=7\times\frac{3}{35}\)
\(A=\frac{3}{5}\)
2) \(B=\frac{1}{25\times27}+\frac{1}{27\times29}+\frac{1}{29\times31}+...+\frac{1}{73\times75}\)
\(B=\frac{1}{2}\times\left(\frac{2}{25\times27}+\frac{2}{27\times29}+\frac{2}{29\times31}+...+\frac{2}{73\times75}\right)\).
\(B=\frac{1}{2}\times\left(\frac{1}{25}-\frac{1}{27}+\frac{1}{27}-\frac{1}{29}+\frac{1}{29}-\frac{1}{31}+...+\frac{1}{73}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\times\left(\frac{1}{25}-\frac{1}{75}\right)\)
\(B=\frac{1}{2}\times\frac{2}{75}\)
\(B=\frac{1}{75}\)
3) \(C=\frac{4}{2\times4}+\frac{4}{4\times6}+\frac{4}{6\times8}+...+\frac{4}{2008\times2010}\)
\(C=\frac{4}{2}\times\left(\frac{2}{2\times4}+\frac{2}{4\times6}+\frac{2}{6\times8}+...+\frac{2}{2008\times2010}\right)\)
\(C=2\times\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2008}-\frac{1}{2010}\right)\)
\(C=2\times\left(\frac{1}{2}-\frac{1}{2010}\right)\)
\(C=2\times\frac{502}{1005}\)
\(C=\frac{1004}{1005}\)
_Chúc bạn học tốt_