A class collects $50 to buy flowers for a classmate who is in the hospital. Roses cost $3 each, and carnations cost $2 each. No other flowers are to be used. How many different bouquets could be purchased for exactly $50?_______
设集合,,则.
设,且,则的最小值是.
已知直线的方向向量分别为,若,则实数=.
“两条直线不相交”是“两条直线是异面直线”的条件.(填“充分不必要”、“必要不充分”、“充要”、“既不必要又不充分”中的一个)
双曲线的渐近线方程为.