题目

阅读理解 Types of Fire Extinguishers Fire extinguishers are marked according to the kind of fore on which they should be used. Using the wrong type of the extinguishers can be dangerous and make things worse. Type A extinguishers are used for fires on paper, cloth, wood, rubber, many plastics and so on. These types of fires usually leave ashes when they burn, so use Type A extinguishers for fires that leave ashes. Type B extinguishers are used for burning liquids, such as oil, gasoline, and paints. These substances often come in barral-shaped containers, so use Type B extinguishers for suc写出下列命题的否定，并判断其真假：（1）p：不论m取何实数，方程x2+mx-1=0必有实数根；（2）p：有些三角形的三条边相等；（3）p：菱形的对角线互相垂直；（4）p：存在一个实数x，使得3x＜0．