七年级数学 解方程组(加减法3) 课件.pptVIP

二元一次方程组的解法 七年级数学(下) 作业 * * * 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× (