Rút gọn biểu thức:a)(2a+b+3c)-(a-b+c) b)(a+b-c)+(a-b_+c)-(a-b-c) c)(a-2b-c)+(-2a+b-c)-(-a-b-2c)

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1. a)(2a+b+3c)-(a-b+c)

   =2a+b+3c-a+b-c

   =(2a-a)+(b+b)+(3c-c)

   =a+2b+2c

   b)(a+b-c)+(a-b+c)-(a-b-c)

   =a+b-c+a-b+c-a+b+c

   =(a+a-a)+(b-b+b)+(-c+c+c)

   =a+b+c

   c)(a-2b-c)+(-2a+b-c)-(-a-b-2c)

   =a-2b-c+-2a+b-c+a+b+2c

   =(a-2a+a)+(-2b+b+b)+(-c-c+2c)

   =0+0+0

   =0

   Học tốt

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2. a) $( 2a + b + 3c ) – ( a – b + c ) $
   $= 2a + b + 3c – a +b – c$
   $= ( 2a – a ) + ( b + b ) + ( 3c – c )$ 
   $= a + 2b + 2c$

   b) $( a + b + c ) + ( a – b + c ) – ( a – b – c ) $
   $= a + b + c + a – b + c – a + b + c$
   $= ( a + a – a ) + ( b – b + b ) + ( c + c + c )$ 
   $= a + b + 3c$

   c) $( a -2b – c ) + ( -2a + b – c ) – ( -a – b – 2c )$
   $= a – 2b – c – 2a + b – c + a + b + 2c$
   $= ( a- 2a + a ) + ( -2b + b + b ) + ( -c -c + 2c )$ 
   $= 0 + 0 + 0 $
   $= 0 $

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