Love lasts a lifetime

During He Yu's lecture, he saw some teachers pointing fingers.

He wondered if he had made any mistakes in his lecture.

Or is it the student's answer and what's wrong with the calculation formula written on the blackboard?

He thought for a moment. I feel that the class I conducted today was more satisfactory than usual, and the calculation formulas written by the students on the blackboard were not only accurate, but also very beautifully written.

He calmed down for a while and looked at his watch. The class only lasted fifteen minutes.

The remaining time is quite sufficient.

He continued steadily and calmly;"Students!

Please read the formula for calculating the angle between two straight lines carefully, and think about and study the instructions on how to calculate in the textbook.

Here you go Two minutes."

While the students were reading the textbook, he walked steadily to see if any of the students had written and drawn around the calculation formulas before class.

He read it over and was very satisfied.

The students followed his usual requirements.

He looked at his watch. The student had only been reading for two minutes.

He began to ask questions;"Now, what does tanθ in the angle formula mean?

Please answer the students behind the first row."

The female student immediately stood up and replied:"It is the tangent of the angle θ between two straight lines.

And it must be It is correct.

The answer is completed. Is it correct? Please correct me!"

"Ask a previous classmate to comment. What was his answer? ," the student who was called by He Yu quickly stood up and said,"I think she not only answered correctly, but also explained that the tangent value of the angle θ between the two straight lines must be positive."

"Why do you say that not only did she answer correctly, but she also said that the tangent of the angle θ between two straight lines must be positive?"He Yu asked

"Because the absolute value symbol is added after the equal sign in the calculation formula"

"I think your answers are very satisfactory!

I wonder what my colleagues think of my evaluation?

A male student stood up without raising his hand and said,"Teacher He!"

I very much agree with your evaluation, but I would like to ask, why does the tangent of the angle θ have to be positive?

Please ask their two classmates to answer?"

……..

The two classmates were silent for a while and no one answered.

One of the classmates said;"Sorry!

I just saw an absolute value sign added after the equal sign, so the tangent value tanθ must be positive.

As for why it must be positive?

I have not studied it carefully.

If you know, please tell me Me!"

The student who was asked the question said:"Because only when you find the positive value of tanθ, you can find out the angle θ corresponding to tanθ in the trigonometric function table." At this time, He Yu asked"The students said, is this student's answer correct?"

A female classmate rushed to answer and said:"He is very right. When I was studying why tanθ should take a positive value, I only noticed that there is an absolute value after the equal sign. Symbols."

He Yu said;"Students can acquire knowledge by studying each other in this way, which shows that your self-study ability has improved.

Only in this way can you develop your intelligence. It is much better than me telling you directly.

But. Let me ask further, except when tanθ takes a positive value, we can find out the angle θ corresponding to tanθ in the trigonometric function table.

But why doesn’t it work when tanθ takes a negative value?"

……..

No one answered at this time.

Suddenly a student stood up and said:"Teacher He!

I guess if tanθ takes a negative value, it means that the angle between the two straight lines does not exist?""

"Students, think about it, does his guess make sense?"

……..

No one answered!

It’s true; teachers don’t lecture in class, they just let students explore.

It's so strange that it's hard to figure out.