Bài `3:`
`a, x^2=81`
`=> x^2 in {9^2; (-9)^2}`
`=> x in {9;(-9)}`
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`b, x^2=16/25`
`=> x^2 in {((4^2)/(5^2));((-4)^2/(-5)^2)}`
`=> x in {4/5; ((-4)/-5)}`
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`c, (x-0,5)^2=9/4`
`=> (x-0,5)^2 in {(3^2/2^2); ((-3)^2/(-2)^2)}`
`=> x-0,5 in {3/2; (-3)/(-2)}`
`=> x in {2}`
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`d, 11/x=(2,5)/4`
`=> 11.4=2,5.x`
`=> 44=2,5.x`
`=> x=44:2,5`
`=> x=17,6`
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`e, (3x)/15=(2,2)/(-1,1)`
`=> 3x.(-1,1)=2,2.15`
`=> 3x.(-1,1)=33`
`=> 3x=33:(-1,1)`
`=> 3x=-30`
`=> x=(-30):3`
`=> x=-10`
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`f, (0,1)/(4,5x)=0,2`
`=> 4,5x=0,1:0,2`
`=> 4,5x=0,5`
`=> x=0,5:4,5`
`=> x=1/9`
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`g, (1 1/2)/(2,5)=(1/5)/x`
`=> (3/2)/(5/2)=(1/5)/x`
`=> 3/5=(1/5)/x`
`=> x=1/5:3/5`
`=> x=1/5xx5/3`
`=> x=1/3`
