七年级数学解方程组7

3x-5y = 6,
①
x+4y = -15. ② 解 ②×3,得 3x+12y = -45 3x - 5y = 6 17y = -51, 即 y = -3. 将y=-3代入②,得 x+4× ( -3 )= -15, x-12= -15, 即 x = -3 所以 x = -3, y = -3. ③ ①
做一做:用加减法解方程组
x 3 y 2 (1) 2 x y 18 2 x y 0, (2) 4 x 3 y 6.
x 8, y 2. x 3, y 6.
解方程组: (6) 解
3x - 4y = 10, ① 5x+6y = 42. ②
消去x
③ - ①,得 12y-(-5y) = -45-6,
解方程组: (4) 解
① 3x+7y = 100. ②
③ ② 3x +7y = 100.
x-3y = -20,
消去x
①×3,得 3x-9y = -60,
② - ③,得7y-(-9y) = 100-(-60), 16y = 160, 即 y =10. 将y=10代入①,得 x-3× 10 = -20 x-30 = -20, 即 x =10. x = 10, 所以 y = 10.
消去y
③ ④
① ×3,得 9x - 12y = 30, ② ×2,得 10x+12y = 84. ③+ ④,得 19x = 114,
x = 6. 即 把x=6代入②,得 5×6+6y = 42, 30+6y = 42, 6y = 42-30, 6y = 12, 即 y = 2. x = 6, 所以 y = 2.
解方程组: (9) 解 由②,得
2x-7y =10, ① 3x-8y- 10 = 0. ② 3x-8y = 10. ④ ⑤ ③
消去x
③×2,得 6x-16y = 20, ①×3,得 6x-21y = 30.
④- ⑤,得 -16y-(-21y) = 20-30, 5y = -10, y = -2. 把y=-2代入①,得 2x-7( -2 ) = 10, 2x+14 = 10, x = -2, 所以 y = -2.
解方程组: (5) 解
4x-2y =14, 5x+y = 7.
① ② ③ ①
消去y
②×2,得 10x+2y = 14, 4x - 2y = 14. ③+①,得 10x+4x = 14+14, 14x = 28, 即 x = 2.
将x=2代入②,得 5×2+y = 7, 10+y = 7, 即 y = -3. 所以 x = 2, y = -3.
解方程组: (8) 解
2x - 3y =8, ① 5y-7x = 5. ② ③ ④
消去y
① ×5,得 10x - 15y =40, ② ×3,得 15y-21x = 15. ③+④,得 -11x = 55,
x = -5. 即 把x=-5代入②,得 5y-7×(-5 ) = 5, 5y+35 = 5, 5y = 5-35, 5y = -30, 即 y = -6. x = -5, 所以 y = -6.
所以
x=-1,
解方程组: (2) 解 ①- ②,得
5x+2y =1, 3x+2y = 3.
① ②
消去y
5x-3x = 1-3, 2x = -2,
即
x = -1.
2y = 3+3 2y = 6, y= 3. 即 x = -1, y = 3.
将x= -1代入②,得 3×(-1) +2y = 3,
所以
解方程组: (3)
x 3 y 20 0, (附加题) (3)
3x 7 y 100 0.
2x = 10-14, 2x = -4, 即 x = -2.
做一做:用加减法解方程组
2 x 4 y 6, (1) 3x 2 y 11. 2 x 3 y 7, (2) 3x 5 y 1.
x 5, y 1. x 2, y 1. x 10, y 10.
加减法(2)
七年级数学(下)
解方程组: (1) 解
7x-2y=3, ① 9x+2y= -19. ②
消去y
①+ ②,得 7x+9x=3+(-19), 16x= -16, 即
x= -1.
2y = -19+9 即 2y = 百度文库10, y= -5. y= -5.
将x= -1代入②,得 9×(-1) +2y = -19,
解方程组: (7) 解
3x - 2y = 6, ① 2x+3y = 17. ② ③ ④
消去y
① ×3,得 9x - 6y = 18, ② ×2,得 4x+6y = 34. ③+ ④,得 13x = 52,
x = 4. 即 把x=3代入②,得 2×4+3y = 17, 8+3y = 17, 3y = 17-8, 3y = 9, 即 y = 3. x = 4, 所以 y = 3.