数学：7.2《二元一次方程组的解法》(加减法3)课件（华师大版七年级下）

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二元一次方程组的解法
七年级数学(下)
加减法(3)
解方程组:
3x - y = 3,
4x+3y = 17.
②
①
③+②,得
解
①×3,得
③
x = 2.
即
即
3×2-y = 3,
y = 3.
所以
x = 2,
y = 3.
9x-3y = 9,
4x +3y = 17.
②
13x = 26,
将x=2代入①,得
6-y = 3,
-y = 3-6.
(1)
消去y
或将x=2代入②,得
4×2+3y = 17,
8+3y = 17
3y =17-8,
3y = 9,
y = 3.
解方程组:
3x - y =3,
4x + 3y = 17.
②
①
④ - ③,得
解
①×4,得
③
y = 3.
即
即
3x-3 = 3,
所以
x = 2,
y = 3.
12x - 4y = 12,
12x + 9y = 51.
④
9y-(-4y) = 51-12,
将y = 3代入①,得
3x = 3+3,
x = 2 .
(1)
消去x
②×3,得
13y = 39,
解方程组:
3x - 4y = 10,
5x+6y = 42.
②
①
解
x = 6.
即
即
所以
x= 6,
y= 2.
19x = 114,
把x=6代入②,得
y= 2.
① ×3,得
② ×2,得
(2)
9x - 12y = 30,
10x+12y = 84.
③
④
③+ ④,得
5×6+6y = 42,
30+6y = 42,
6y = 42-30,
6y = 12,
消去y
解方程组:
3x - 4y = 10,
5x+6y = 42.
②
①
解
y = 2.
即
即
所以
x= 6,
y= 2.
38y = 76,
把y= 2代入①,得
x= 6.
① ×5,得
② ×3,得
(2)
15x - 20y = 50,
15x+18y = 126.
③
④
④- ③,得
3x - 4×2 = 10,
3x-8 = 10,
3x = 10+8,
3x= 18,
消去x
解方程组:
3x - 2y = 6,
2x+3y = 17.
②
①
解
x= 4.
即
即
所以
x = 4,
y = 3.
13x = 52,
把x= 4代入②,得
y = 3.
① ×3,得
② ×2,得
(3)
9x - 6y = 18,
4x+6y = 34.
③
④
④ + ③,得
2 × 4 + 3 y = 17,
8 + 3y= 17,
3y = 17-8,
3y = 9,
消去y
解方程组:
3x - 2y = 6,
2x+3y = 17.
②
①
解
y = 3.
即
即
所以
x = 4,
y = 3.
13y = 39,
把y=3代入②,得
x = 4.
① ×2,得
② ×3,得
(3)
6x - 4y = 12,
6x+9y = 51.
③
④
④ - ③,得
2x+3 ×3 = 17,
2x + 9= 17,
2x = 17-9,
2x = 8,
消去x
解方程组:
2x - 3y = 8,
5y-7x = 5.
②
①
解
x = -5.
即
即
所以
x = -5,
y = -6.
-11x = 55,
把x=-5代入②,得
y = -6.
① ×5,得
② ×3,得
(4)
10x - 15y = 40,
15y-21x = 15.
③
④
③+④,得
5y-7×(-5) = 5,
5y+35 = 5,
5y = 5-35,
5y = -30,
消去y
解方程组:
2x - 3y = 8,
5y -7x = 5.
②
①
解
x = -5.
即
即
所以
x = -5,
y = -6.
-11x = 55,
把x=-5代入②,得
y = -6.
① ×5,得
② ×3,得
(4)
10x - 15y = 40,
-21x+15y = 15.
③
④
③+④,得
5y-7×(-5) = 5,
5y+35 = 5,
5y = 5-35,
5y = -30,
消去y
原方程组可化为
2x - 3y = 8,
-7x+5y = 5.
解方程组:
2x - 3y = 8,
5y-7x = 5.
②
①
解
y = -6.
即
即
所以
x = -5,
y = -6.
-11y = 66,
把y= - 6代入①,得
x = -5.
① ×7,得
② ×2,得
(4)
14x - 21y = 56,
-14x+10y = 10.
③
④
③+④,得
2x - 3×(-6) = 8,
2x+18 = 8,
2x = 8-18,
2x = -10,
消去x
原方程组可化为
2x - 3y = 8,
-7x+5y = 5.
解方程组:
2x-7y = 10,
3x-8y- 10 =0.
②
①
解
2x-7× ( ) = 10,
5y = -10,
即
所以
x = -2,
y = -2.
2x+14=10,
把y=-2代入①,得
y = -2.
(5)
3x-8y = 10.
③
② ×2,得
①×3,得
6x-16y = 20,
6x-21y = 30.
④
④- ③,得
-16y-(-21y) = 20-30,
-2
2x =10-14,
2x = -4,
x = -2.
消去x
原方程组可化为
2x-7y = 10,
解二元一次方程组的基本思想是
________,化_____为_________;
基本方法有________、 ________.
消元
代入法
加减法
二元
一元
做一做:用加减法解方程组
(附加题)
2
-3
1
想一想
作业