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)\tan x + \cot x = 2(\sin 2x + \cos 2x).

2. Solution Steps

First, we can rewrite tanx\tan x and cotx\cot x in terms of sinx\sin x and cosx\cos x:
tanx=sinxcosx\tan x = \frac{\sin x}{\cos x} and cotx=cosxsinx\cot x = \frac{\cos x}{\sin x}.
Thus, the equation becomes
sinxcosx+cosxsinx=2(sin2x+cos2x)\frac{\sin x}{\cos x} + \frac{\cos x}{\sin x} = 2(\sin 2x + \cos 2x).
Combining the fractions on the left side gives
sin2x+cos2xsinxcosx=2(sin2x+cos2x)\frac{\sin^2 x + \cos^2 x}{\sin x \cos x} = 2(\sin 2x + \cos 2x).
Since sin2x+cos2x=1\sin^2 x + \cos^2 x = 1, we have
1sinxcosx=2(sin2x+cos2x)\frac{1}{\sin x \cos x} = 2(\sin 2x + \cos 2x).
We can rewrite sinxcosx\sin x \cos x as 12sin2x\frac{1}{2}\sin 2x, so the equation becomes
112sin2x=2(sin2x+cos2x)\frac{1}{\frac{1}{2} \sin 2x} = 2(\sin 2x + \cos 2x).
This simplifies to
2sin2x=2(sin2x+cos2x)\frac{2}{\sin 2x} = 2(\sin 2x + \cos 2x).
Divide both sides by 2:
1sin2x=sin2x+cos2x\frac{1}{\sin 2x} = \sin 2x + \cos 2x.
Multiply both sides by sin2x\sin 2x:
1=sin22x+sin2xcos2x1 = \sin^2 2x + \sin 2x \cos 2x.
Recall the identity sin22x+cos22x=1\sin^2 2x + \cos^2 2x = 1. Thus, we can write
sin22x+sin2xcos2x=sin22x+cos22x\sin^2 2x + \sin 2x \cos 2x = \sin^2 2x + \cos^2 2x.
Subtracting sin22x\sin^2 2x from both sides gives
sin2xcos2x=cos22x\sin 2x \cos 2x = \cos^2 2x.
Then, cos22xsin2xcos2x=0\cos^2 2x - \sin 2x \cos 2x = 0.
Factoring out cos2x\cos 2x gives
cos2x(cos2xsin2x)=0\cos 2x(\cos 2x - \sin 2x) = 0.
This implies cos2x=0\cos 2x = 0 or cos2x=sin2x\cos 2x = \sin 2x.
If cos2x=0\cos 2x = 0, then 2x=π2+kπ2x = \frac{\pi}{2} + k\pi for some integer kk, so x=π4+kπ2x = \frac{\pi}{4} + \frac{k\pi}{2}.
If cos2x=sin2x\cos 2x = \sin 2x, then tan2x=1\tan 2x = 1, so 2x=π4+kπ2x = \frac{\pi}{4} + k\pi for some integer kk, so x=π8+kπ2x = \frac{\pi}{8} + \frac{k\pi}{2}.
We must check if these solutions are valid. If x=π4+kπ2x = \frac{\pi}{4} + \frac{k\pi}{2}, then 2x=π2+kπ2x = \frac{\pi}{2} + k\pi, so sin2x=±1\sin 2x = \pm 1 and cos2x=0\cos 2x = 0. If sin2x=1\sin 2x = 1, the original equation is satisfied. If sin2x=1\sin 2x = -1, the left side of the original equation is 2sin2x=2\frac{2}{\sin 2x} = -2, while the right side is 2(sin2x+cos2x)=2(1+0)=22(\sin 2x + \cos 2x) = 2(-1 + 0) = -2, so the equation is satisfied.
If x=π8+kπ2x = \frac{\pi}{8} + \frac{k\pi}{2}, then 2x=π4+kπ2x = \frac{\pi}{4} + k\pi, so sin2x=cos2x=±22\sin 2x = \cos 2x = \pm \frac{\sqrt{2}}{2}. If sin2x=cos2x=22\sin 2x = \cos 2x = \frac{\sqrt{2}}{2}, then 2sin2x=22/2=22\frac{2}{\sin 2x} = \frac{2}{\sqrt{2}/2} = 2\sqrt{2}, and 2(sin2x+cos2x)=2(22+22)=222(\sin 2x + \cos 2x) = 2(\frac{\sqrt{2}}{2} + \frac{\sqrt{2}}{2}) = 2\sqrt{2}. If sin2x=cos2x=22\sin 2x = \cos 2x = -\frac{\sqrt{2}}{2}, then 2sin2x=22/2=22\frac{2}{\sin 2x} = \frac{2}{-\sqrt{2}/2} = -2\sqrt{2}, and 2(sin2x+cos2x)=2(2222)=222(\sin 2x + \cos 2x) = 2(-\frac{\sqrt{2}}{2} - \frac{\sqrt{2}}{2}) = -2\sqrt{2}.
Therefore, the general solutions are x=π4+kπ2x = \frac{\pi}{4} + \frac{k\pi}{2} and x=π8+kπ2x = \frac{\pi}{8} + \frac{k\pi}{2}, where kk is an integer.
Note, however, that sinx\sin x and cosx\cos x cannot be zero. So tanx\tan x and cotx\cot x must be defined.

3. Final Answer

x=π8+kπ2x = \frac{\pi}{8} + \frac{k\pi}{2} and x=π4+kπ2x = \frac{\pi}{4} + \frac{k\pi}{2}, where kk is an integer.

Related problems in "Trigonometry"

We are asked to simplify the expression: $\frac{\sin x + \sin 2x + \sin 3x}{\cos x + \cos 2x + \cos ...

TrigonometryTrigonometric IdentitiesSum-to-Product FormulasTangent FunctionEquation Solving
2025/4/22

The problem asks to solve for $x$ in the equation $\sin(5 \times 21^{\circ}) = \cos(9^{\circ}) + \ta...

Trigonometric EquationsTrigonometric IdentitiesSineCosineTangentAngle Sum and Difference Identities
2025/4/21

The problem is to simplify the expression $\tan(x) \cot(9^\circ) + \tan(5x)$. I assume the question...

Trigonometric IdentitiesTangent FunctionCotangent FunctionSimplification
2025/4/21

We are asked to simplify the expression $\frac{2 \tan 60^{\circ} + \cos 30^{\circ}}{\sin 60^{\circ}}...

TrigonometryTrigonometric IdentitiesSimplification
2025/4/21

The problem asks us to evaluate several trigonometric expressions: a. $sin(390^\circ)$ b. $cos(\frac...

TrigonometryTrigonometric FunctionsSineCosineTangentAngle ReductionUnit Circle
2025/4/19

We are asked to prove the trigonometric identity $\sin^4 A - \cos^4 A = 2\sin^2 A - 1$.

Trigonometric IdentitiesPythagorean IdentityDifference of SquaresAlgebraic Manipulation
2025/4/15

We are given the equation $\cos(4x) = \sin(x)$ and we need to solve it in the interval $[-\frac{\pi}...

Trigonometric EquationsTrigonometric IdentitiesSolving EquationsIntervalSineCosineRoots
2025/4/14

We need to prove two trigonometric identities. 1. $\frac{2 + \sin 2x - 2 \cos 2x}{1 + 3 \sin^2 x - \...

Trigonometric IdentitiesDouble Angle FormulasHalf Angle FormulasTrigonometric Simplification
2025/4/14

The problem asks us to solve several trigonometric equations in $\mathbb{R}$ and represent the solut...

Trigonometric EquationsUnit CircleTrigonometric IdentitiesSolving Equations
2025/4/14

We are asked to solve the following trigonometric equations in $R$ and represent the solutions on th...

Trigonometric EquationsUnit CircleTrigonometric IdentitiesSolutions
2025/4/14