This is equal to the sine of theta.

两侧乘重的大小。

So this is 90 minus theta degrees.

这个也是90-θ度。

So this is going to be equal to the sine of theta.

两侧乘重的大小。

This too will be 90 minus theta degrees.

再看如何求出这个角。

A change from alpha wave drowsiness to lower frequency theta brain waves signify early sleep.

从α波睡意到较低频率的脑电波的变化表示早睡。

There are three frequencies that we need to understand, Beta frequency, Theta frequency, and Alpha frequency.

我们需要理解三种频率，贝塔频率，西塔频率和阿尔法频率。

And that's why it's cosine of theta.

因此 两者相等。

This right here is 90 minus theta.

那么上面这个角是多少。

Or, if this is theta, this angle right over here is going to be 90 minus theta.

那么这个角就是90-θ另外一个几何知识 不知道你们记不记得。