3. Given that there were 4 Heads in the first 7 tosses, find the probability that the 2nd Heads occurred at the 4th toss. Give a numerical answer.[填空1]

Well, let’s put our heads together and find ___to the problem. Science and engineering students often _____ their heads in books instead of going to dances and cinema. Well, let’s put our heads together and find_________________ to the problem. Well,let′s put our heads together and find a（n）__________to the problem. Well, let's put our heads together and find a(n)（）to the problem. Well,let’s put our heads together and find a(n)________to the problem. When you have difficulty, do remember to _________________. Two heads are better than one. When you have difficulty, do remember to _________. Two heads are better than one. Heads from Asia and Europe attended the conference and President Hu ________ a speech on thefirst day.[ ] The surface of the Seismograph was cast with ____ dragons, whose bodies were downward vertically and heads pointed to ____directions. e are many students of Grade Threereading in the reading room, most of _________ with their heads buried in theirreference books and notebooks re are many students of Grade Threereading in the reading room, most of _________ with their heads buried in theirreference books and notebooks. Even in winter the rocame to feed on the berries and on the seed heads ofthe dried weeds rising above the snow. 有25 个工人看管机器台数资料如下: 3,4,4,2,2,2,5,4,4,3,4,3,4,4,2,2,4,3,4,6,3,4,5,2,4。如按以上资料编制分配数列,应采用（ ）。 计算：(1)-34   (2)-(-3)3  (3)-(-2/3)4(4)(4/5)2  (5)-3/22  (6)-(-2/5)3 设向量组α1=（1，2，3，4），α2=（2，3，4，5，），α3=（3，4，5，6），α4=（4，5，6，7），则秩（α1，α2，α3，α4）=____. 有以下程序 #include void main() { int i, j; for(i=3; i>=l; i--) { for(j=1; j People who ______ athletic sports are generally believed to _____ strong bodies, firm muscles but dull heads, which of course is not the case. [ ] We are all curious about what the astronauts have to____ when they try to drink waterwhile standing ontheir heads.[ ] The wonderful world which lies before us today has been _________ by hardworking men with clear heads and inventive genius.