[Index]
Introduction
Essence
Memorizing
Understanding
Summarizing
Examples
Example 1
Example 2
Example 3
Example 4
Example 5
Example 6
Example 7
Example 8
General ideas
Exercises
Final thoughts
|
Example 8
Suppose that a, b, c, 
,
and 
are real numbers, with the following conditions: 
,
,
k
is any integer.
 |
(1) |
 |
(2) |
Prove that 
.
You might already notice that this is the hardest problem in our entire Learning section. Yes, it's a difficult problem. Especially you might not even know where you should start.
Like the previous one, this is a conditional identity too. We see this problem gives us many more conditions. How do we start? What formulas can help us to prove it? We believe that every problem has some clues. Which tell us how to solve it. Let's see what clues this problem has.
From the left side of the goal identity, we see the angle 
,
which let us believe it must be relate to the Sum
to Product formulas. So let's try to add equations (1) and
(2) by using the Sum to Product
formulas. We'll get the angle
and see if we can get anything else.
| (1) + (2) |  |
(3) |
From (3), we use the Sum to Product formulas to get:
Then we divide 2 from both sides and take out the common factor:
 |
(4) |
Let's look at equation (4), because the goal identity has square
at both sides, so if we want to get 
we need square both sides of equation (4).
 |
(5) |
