一道数学题

显示全部楼层 · 2009-8-4 09:54:56

已知甲独做4天，完成1/3，则甲每天做1/12，设这段路全长为x米，则（1-1/3)x=(1/12x+75)*3，解得x=540米

千问 · 2009-8-4 09:54:56

4x = 1/3 L (1)3(x + y) = 2/3L (2)y = 75 (3)(3) 代入(2)-> 3(x + 75) = 2/3L -> (x + 75) = 2/9L-> 4(x + 75) = 8/9L -> L = (4x+4*75) * 9/8 = 4x * 9/8 + 75*9/2-> L = (1/3)L * 9/8 + 75*9/2 = 3L / 8 + 75*9/2-> 5/8L = 75*9/2 -> L = 75 * 9 * 8 / 5 / 2 = 15 * 9 * 4 = 60 * 9 -> L = 540

千问 · 2009-8-4 09:54:56

1.4x = 1/3 L (1)3(x + y) = 2/3L (2)y = 75 (3)(3) 代入(2)-> 3(x + 75) = 2/3L -> (x + 75) = 2/9L-> 4(x + 75) = 8/9L -> L = (4x+4*75) * 9/8 = 4x * 9/8 + 75*9/2-> L = (1/3)L * 9/8 + 75*9/2 = 3L / 8 + 75*9/2-> 5/8L = 75*9/2 -> L = 75 * 9 * 8 / 5 / 2 = 15 * 9 * 4 = 60 * 9 -> L= 540 2.设甲每天修x米,则总长12x米。 4x+3(x+75）=12x
x=45则总长12x=540米了

千问 · 2009-8-4 09:54:56

设：甲的效率为X/天,公路全长为Y米.可得方程组{4X=2/3Y
{3(X+75)=2/3Y得｛X=45
｛Y=540答：这段路的全长为540米.