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1、第 7章 二元一次方程组加减法 (2)解方程组 : 7,9x+2y= 解y= (+2y = x= 1,y= + ,得将 x= ,得(1)7x+9x=3+(16x= y = 去 5x+2y =1,3x+2y = 3. 解y= (+2y = 3,x = -1,y = - ,得将 x= ,得(2)5 1x = y = 3+ 6,消去 3 6,x+4y = - ,得解 3,得 17y = y = ( )= 3x = x = -3,y = 2y = 5y = 6 125y) = y= ,得3)消去 x+7y = 100. - ,得解 3,得 16y = 160,y = x = 10,y = x +7y
2、= 100. 79y) = 100-(将 y=10代入 ,得 4)消去 414,5x+y = 7. + ,得解 2,得 14x = 28,x = 2+y = 7,y = x = 2,y = y = 14,4x - 2y = 14. 10x+4x = 14+14,将 x=2代入 ,得10+y = 7,(5)消去 用加减法解方程组18223)1(2)2( 3x - 4y = 10,5x+6y = 42. 解x = x = 6,y = 114,把 x=6代入 ,得y = 2. 3,得 2,得(6)9x - 12y = 30,10x+12y = 84. + ,得5 6+6y = 42,30+6y =
3、42,6y = 42y = 12,消去 3x - 2y = 6,2x+3y = 17. 解x = x = 4,y = 52,把 x=3代入 ,得y = 3. 3,得 2,得(7)9x - 6y = 18,4x+6y = 34. + ,得2 4+3y = 17,8+3y = 17,3y = 17y = 9,消去 2x - 3y =8,5 5. 解x = x = -5,y = 55,把 x= ,得y = 5,得 3,得(8)10x - 15y =40,15 15. + ,得5( = 5,5y+35 = 5,5y = 5y = 去 210,30 = 0. 解2 ) = 10,5y = 所以 x = -2,y = 4 = 10,把 y=,得y = 9)3 ,得 2,得 3,得6 20,6 30. - ,得 21y) = 202 2x = 10x = -4,x = 用加减法解方程组42)1(32)2(203)3(0加题 )
