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câu b )
ta phân phối 2 vô
=> \(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+\frac{2}{17.19}+\frac{x}{209}=\frac{10}{209}\)
\(\Rightarrow\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{17}-\frac{1}{19}+\frac{x}{209}=\frac{10}{209}\)
\(\Rightarrow\frac{8}{209}+\frac{x}{209}=\frac{10}{209}\)
\(\Rightarrow\frac{x}{209}=\frac{2}{209}\)
\(\Rightarrow x=2\)
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(x+1)+(x+2)+(x+3)+...+(x+52)=780
=>(x+x+x..+x)+(1+2+3+....+52)=780
52 số x 52 số
=>52x+(52+1)x52:2=780
=>52x+1378=780
=>52x=-598
=>x=-119,6
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Ta có X + X = 52
X x 2 = 52
X = 52 : 2
X = 26
Ta có: 30% = 30/100
30% x là 26 x 30/100 = 7,8
Vậy 30% x là 7,8
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x + 3 + x + 5 + x + 7 - x - 9 = 52
2x + 6 = 52
2x = 46
x = 23
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x : 1/3 + x : 1/4 + x : 1/5 + x = 52
x * 3 + x * 4 + x * 5 + x = 52
x * (3 + 4 + 5 + 1) = 52
x * 13 = 52
x = 52 : 13 = 4
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124 x 52 - 124 x 42
=124x(52-42)
=124x10
=1240
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a) 75% x X+ 3/4 = 30
75%x X= 30-3/4
75%x X=117/4
X=117/4 : 75/100
X=117/4 x 100/75
X= 39
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\(\left(52-x\right):3=5,6\\ 52-x=5,6\times3\\ 52-x=16,8\\ x=52-16,8\\ x=35,2\)
( x + x * 3 ) : 52 = 209
4x = 209 * 52
4x = 10868
x = 2717