câu 1: a) -50+20 b)-30-10 c)-55+(-20) d) -99 +100-1+99 câu 2: Bỏ ngoặc

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1. câu 1

   a) -50+20=-30

   b)-30-10=-40

   c)-55+(-20)=-75

   d)-99+100-1+99=(-99+99)+(100-1)=0+99=99

   câu 2

   a)-50-[-30-(20-30)-50]

   =50-(-30)-20+30-50

   =80-20+30-50

   =60+30-50

   =90-50

   =40

   b)25+(30-25)-30

   =25+30-25-30

   =(25-25)+(30-30)

   =0+0=0

   câu 3

   a)25+x=30-(-15)

   25+x=45

   x=45-25

   x=20

   b)x+20=-50+25

   x+20=-25

   x=-25-20

   x=-45

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2. câu 1 :

   a) -50+20 

   = – ( 50 – 20 )

   = – 30

    

   b) – 30 – 10

   = ( – 30 ) + ( -10 )

   = -40

   c) – 55 + ( – 20 )

   = – 75

   d) -99 +100-1+99       

   = ( – 99 + 99 ) + ( 100 – 1 )

   =   0 + 99

   = 99

   câu 2 :

   a)- 50 – [ – 30 – ( 20 – 30 ) – 50 ]

   = – 50 + 30 + ( 20 – 30 ) + 50

   = -50 + 30 +  20 – 30  + 50

   = ( -50 + 50 ) + ( 30 – 30 ) + 20

   = 20

   b) 25 + ( 30 – 25 ) – 30  

   = 25 + 30 – 25 – 30

   = ( 25 – 25 ) + ( 30 – 30 )

   = 0

   câu 3 :

   a) 25 + x = 30  – ( – 15 )

       25 + x = 30 + 15

       25 + x = 45

               x = 45 – 25

               x = 20

    

   b) x + 20 = -50 + 25

       x + 20 = – 25

       x          = -25 – 20

       x          = – 45

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