giúp mk vs các bn iu ưi, sắp thi rùi :((( a). x/4 = 9/x b). ( -1/2x + 2/3 ) : -5/7 = 28/15

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1. a)x/4=9/x

   x*x=4*9

   x^2=36

   x=6

   b)(-1/2x+2/3):-5/7=28/15

   (-1/2x+2/3)*-7/5=28/15

   7/10x-14/15=28/15

   7/10x=28/15+14/15

   7/10x=42/15

   x=42/15*10/7

   x=420/105

   x=4

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