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.
消去y
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.
x 3 y 20 0, (附加题) (3)
3x 7 y 100 0.
华东师大版七年级下册
第7章 二元一次方程组
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 = -10, y= 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.