整式的加减(去括号)课件ppt.ppt

去括号(1) ( ) a b c ? ?温故而知新(2) ( ) a ? ?? ab ac ?a?(4) ( ) a ??? a?(3) ( ) a ??? a?(5) ( ) a ??? a?=( -1)x(3 -7) 复习旧知- ( +5 )= +( +5 )= - ( -7)=+( -7)= - 5 +5 +7 -7 1. 化简=( -1) x 3+ ( -1)x( -7) = - 3 + 7 = 1 x 3+1 x ( -7) = 3 - 7 2. 去括号① -(3 - 7 )② + (3 - 7 ) + 3 与-7的和=( +1 )x(3 -7) 观察这两组算式,看看去括号前后,括号里各项的符号有什么变化? (1) 2( ) (2) 3( ) (3) 4( ) (4) 5( ) a b a b a b a b ? ?? ?? ??? ?? 2 2 3 3 4( ) 4( ) 5( ) 5 a b a b a b a b ??? ??? ?用分配律计算(5) 2( ) (6) 3( ) (7) 4( ) (8) 5( ) a b a b a b a b ? ??? ??? ???? ???( 2) ( 2) ( 3) ( 3)( ) ( 4)( ) ( 4)( ) ( 5)( ) ( 5) a b a b a b a b ? ??? ???? ????? ???想一想根据分配律,你能为下面的式子去括号吗? (1) + ( a-b ) (2) - ( a-b ) = 1x ( a-b ) = ( -1)x( a-b ) 观察这两组算式,看看去括号前后,括号里各项的符号有什么变化? = a-b = - a+b =( -1) xa+(-1) x(-b ) (1) + ( a- b+c ) (2) - ( a- b+c ) = 1x ( a- b+c ) = ( -1)x( a- b+c ) = a- b+c = - a+b-c =( -1) xa+(-1) x(-b)+(-1 ) xc 如果括号外的因数是正数,去括号后原括号内各项的符号与原来的符号相同. 如果括号外的因数是负数,: 去括号,看符号: 是“+”号,不变号; 是“-”号,全变号;原来的符号和括号都扔掉. 练习: (1)去括号: a+(b-c )=———— a+(- b+c )=———— a- ( b-c )=———— a- (- b+c )=————(2)判断正误 a-( b+c )=a- b+c ( ) a-( b-c )=a- b-c ( ) 2b+(-3a+1)=2b-3a-1 ( ) 3a-(3b-c)=3a-3b+c ( ) × ×× a+b-c a- b+c a- b+c a+b-c a- b-c a- b+c 2b-3a+1 √例:为下面的式子去括号= + ( 3a-3b+3c ) = 3a-3b+3c = -3a+3b-3c = - ( 3a-3b+3c ) = +[3 ( a- b+c )] = -[3 ( a- b+c )] (1) +3 ( a - b+c ) (2)- 3 ( a - b+c ) 结论: 括号外面的因数不是 1或-1时, 把符号留在外面,把因数的绝对值按分配率乘进去,最后再去括号. 练习 1:去括号① 9 ( x-z )②-3(- b+c ) ③ 4(-a+b-c )④-7(- x-y+z ) = 9x+9 ×( -z) = 9x- 9z =-[3 ×( -b) +3c] =-(-3b+3c) =3b-3c = 4 ×( -a) +4b+4 ×( -c) = - 4a+4b- 4c = - [7 ( -x) +7 ( -y) +7z] = - (-7x-7y+7z) = 7x+7y-7z