The main cause of global warming is human pollution. Through burning coal, oil and gas, people have been increasing the greenhouse gases in the atmosphere, such as CO2. This adds to the power of the greenhouse effect, making the planet even warmer.

　　Many scientists believe that, if the warming is not stopped, there will be huge climate changes. The sea level could rise by one meter this century.

　　Should this come true, millions of homes will be swallowed up by the sea and the world will be flooded with “climate refugees (难民)” looking for somewhere to live.

　　The average global temperature has increased by almost 0.5 centigrade degree over the past century, scientists expect it to rise by an extra 1～3 degrees over the next 100 years.

　　Warmer weather makes glaciers (冰川) melt, adding more water to the ocean. The warmer temperatures also make water expand, so it takes up more space, causing the sea level to rise. The sea level has risen about 10～25cm in the last 100 years.

　　But Tuvalu is not the first place to face sinking into the sea. Venice, a historic city in Italy best known for its canals, has sunk about 24cm over the past 100 years. Experts say that it will have sunk another 20～50cm by 2050. A century ago, St. Mark’s Square, the lowest point in the city, flooded about nine times a year. Nowadays, it happens more than 100 times.

　　While Venice is slowly sinking into the mud on which it stands, Tuvalu’s rising sea level is caused by global warming.

（2）求证：；

（3）求P_n的表达式．

从原点出发的某质点M，按向量，按向量移动的概率为，设M可到达点（0，n）的概率为P_n．

（1）求P₁和P₂的值；

20．（本小题满分14分）

    （1）求，，，的值，并据此猜测数列的通项公式；

（2）用数学归纳法证明你的猜测．

设．

19．（本题满分14分）  

在四棱锥V-ABCD中，底面ABCD是正方形，侧面VAD是正三角形,平面VAD⊥底面ABCD．

   （1）证明AB⊥平面VAD；

   （2）求面VAD与面VDB所成的二面角的大小．