We are asked to solve the trigonometric equation $\tan x + \cot x = 2(\sin 2x + \cos 2x)$.
TrigonometryTrigonometric EquationsTrigonometric IdentitiesSolution of Equations
2025/4/22
1. Problem Description
We are asked to solve the trigonometric equation tanx+cotx=2(sin2x+cos2x).
2. Solution Steps
First, we can rewrite tanx and cotx in terms of sinx and cosx:
tanx=cosxsinx and cotx=sinxcosx.
Thus, the equation becomes
cosxsinx+sinxcosx=2(sin2x+cos2x).
Combining the fractions on the left side gives
sinxcosxsin2x+cos2x=2(sin2x+cos2x).
Since sin2x+cos2x=1, we have
sinxcosx1=2(sin2x+cos2x).
We can rewrite sinxcosx as 21sin2x, so the equation becomes
21sin2x1=2(sin2x+cos2x).
This simplifies to
sin2x2=2(sin2x+cos2x).
Divide both sides by 2:
sin2x1=sin2x+cos2x.
Multiply both sides by sin2x:
1=sin22x+sin2xcos2x.
Recall the identity sin22x+cos22x=1. Thus, we can write
sin22x+sin2xcos2x=sin22x+cos22x.
Subtracting sin22x from both sides gives
sin2xcos2x=cos22x.
Then, cos22x−sin2xcos2x=0.
Factoring out cos2x gives
cos2x(cos2x−sin2x)=0.
This implies cos2x=0 or cos2x=sin2x.
If cos2x=0, then 2x=2π+kπ for some integer k, so x=4π+2kπ.
If cos2x=sin2x, then tan2x=1, so 2x=4π+kπ for some integer k, so x=8π+2kπ.
We must check if these solutions are valid. If x=4π+2kπ, then 2x=2π+kπ, so sin2x=±1 and cos2x=0. If sin2x=1, the original equation is satisfied. If sin2x=−1, the left side of the original equation is sin2x2=−2, while the right side is 2(sin2x+cos2x)=2(−1+0)=−2, so the equation is satisfied.
If x=8π+2kπ, then 2x=4π+kπ, so sin2x=cos2x=±22. If sin2x=cos2x=22, then sin2x2=2/22=22, and 2(sin2x+cos2x)=2(22+22)=22. If sin2x=cos2x=−22, then sin2x2=−2/22=−22, and 2(sin2x+cos2x)=2(−22−22)=−22.
Therefore, the general solutions are x=4π+2kπ and x=8π+2kπ, where k is an integer.
Note, however, that sinx and cosx cannot be zero. So tanx and cotx must be defined.