1. Right Triangle PQR is to be constructed in the xy plane so that the right angle is at P and PR is parallel to the x-axis. The x-and y- coordinates of P,Q, and R to be integers that satisfy the inequality -4<=x<=5 and 6<=y<=16. How many different triangles with these properties could be constructed? answer: 9900 2. If n is a positive integer less than 200 and (14n)/60 is an integer, then n has how many different positive prime factors? answer: 3 (除了一个一个算,还有什么简单快速的方法吗?) 再请教,做数学时,一题大约分配多长时间。1分种?2 分钟? 多谢 |