(1) c - b = b - a, B为AC的中点, BC = 30 = 20 - b, b = -10b - a = -10 -a = 30, a = -40(2)t秒时, p = -40 - 8tr = -40 + 4tq = 20 - 2tm = (-40 - 8t - 40 + 4t)/2 = -40 -2tn = (-40 + 4t + 20 - 2t)/2 = -10 +...请问有什么问题？"PQ与MN的长存在一个确定的相等关系": 看不出有什么相等关系, 所以怀疑题有问题。这是初一作业，也不是老师出的，应该不会有问题吧……(3)t秒时,p = -40 - 8tq = 20 + 4tn = (0 - 40 - 8t)/2 = -20 - 4tm = (-10 + 20 + 4t)/2 = 5 + 2tPQ = 20 + 4t - (-40 - 8t) = 60 + 12tMN = (5 + 2t) - (-20 - 4t) = 25 + 6tPQ = 2MN + 10或PQ - 2MN = 10我原来理解为比例，用差就通了。