the observer said that the woman was dressed in black clothe

Teaching is learning (made by myself)
Our life has a limit but knowledge not.How much konwledge a man can acquire during his days?Well it depends on his own will.You could be as clever as Einstein but as stupid as a mule which is a matter of your choice.Many people think learning is the best approach to lift himself.That is a good idea.But after you hold much have you thought to share with others?I get one.As we all know,there is a career that links the teaching and learnig together.It is the most glorious occupation under the sun which spreads the fruit of wisdom to common people.I wish to be such a person.Teaching is not a one-way act.When the teacher gives the knowledge to his/her students,the process also helps him/her to consolidate what he/she has learned.An old Chinese saying"look for the old so as to find the new"indicates that teaching is just a way to look back and learning is more important to explore and grip something new.While teaching you may get unexpected surprise that the receiver may tell you something you never heard,If such,teaching becomes another kind of learning and vice verse.So never look down upon anyone who seems inferior to you.It is defenitely a happy experience for everyone to teach and learn.
Thank you!

Observer字面理解就是观察者,其实这是观察者模式.ImageObserver就是Image的观察者,它时刻关注Image是否可用.
很想看到你有的代码

那个俯角是40度和24度吧!
70/tan24度-70/tan40度=73.7998226917,我算的和你一样.估计答案错了.

请贴下上下文
观测者性能研究.供参考

举例：
设声源S,观察者L分别以速度Vs,Vl在静止的介质中沿同一直线同向运动,声源发出声波在介质中的传播速度为V,且Vs小于V,Vl小于V.当声源不动时,声源发现频率为f,波长为X的声波,观察者接受到的声波的频率为：
f'=(V-Vl)V/[(V-Vs)X]=(V-Vl)f/(V-Vs)
所以得 （1）当观察者和波源都不动时,Vs=0,Vl=0,由上式得f'=f
（2）当观察者不动,声源接近观察者时,观察者接受到的频率为
F=Vf/(V-Vs) 显然此时频率大于原来的频率.
由上面的式子可以得到多普勒效应的所有表现.

观察者间的变异性.
这个词主要在科学研究中,当有多个观察者有不同的结论时会用到.