If someone is lost in a forest( 森林 ), he needs only study the trees around him carefully to find his way. On this half of the earth, he will find that the side of the tree with the most leaves and branches(树枝 ) is the south side. Also, by checking the tops of the trees, he will find that they lean(倾斜 ) to the south too.
Another sign( 标志物) for him to know where he is going is found in the bark.( 树皮) That is brighter on the south side.The remanining(树桩 ) of a cut tree is also a finder, for its rings can give him another sign. The rings will be thinner on the south side and thicker on the north side.
One more sign: some very small plants grow at the foot of a tree and they point to the north.
不是纯虚数，则
的取值范围是