根据短文大意选择单词并用其正确形式填入空白处,使补全后的短文意思通顺,语法正确,结构完整.（5分）

walk      way     without     prevent     easy

Trees are useful in three important    1  .The most important way is that they  provide man with food , wood and other products. Trees provide food not only for man, but also for animals.  2   trees, many animals could not live on the earth. It’s not    3  for man to live on the earth, either.

The second important way is that trees give us shade(树荫). On a hot summer day, people are eager to have a rest under the shade of a tree after   4  a long way. You can imagine how important the shade of a tree is to man and animals.

The third important way is that trees help to   5   droughts and flood. However, in many parts of the world, man hasn’t realized it. He has cut trees down in large numbers. In the end, he finds that he has lost the best friends he had.

先化简，再求值：已知a=2，b=-1，求代数式a²b²+3ab-7a²b²-2ab+1+5a²b²的值．