Prove the trigonometric identity: $tan(\frac{\theta}{2}) + cot(\frac{\theta}{2}) = 2csc(\theta)$.

TrigonometryTrigonometric IdentitiesDouble Angle FormulaTangentCotangentCosecantProof
2025/4/28

1. Problem Description

Prove the trigonometric identity: tan(θ2)+cot(θ2)=2csc(θ)tan(\frac{\theta}{2}) + cot(\frac{\theta}{2}) = 2csc(\theta).

2. Solution Steps

We will start with the left-hand side of the equation and try to simplify it to the right-hand side.
tan(θ2)+cot(θ2)tan(\frac{\theta}{2}) + cot(\frac{\theta}{2})
Since cot(x)=1tan(x)cot(x) = \frac{1}{tan(x)}, we can rewrite the expression as:
tan(θ2)+1tan(θ2)tan(\frac{\theta}{2}) + \frac{1}{tan(\frac{\theta}{2})}
Now, let's find a common denominator:
tan2(θ2)+1tan(θ2)\frac{tan^2(\frac{\theta}{2}) + 1}{tan(\frac{\theta}{2})}
We know that 1+tan2(x)=sec2(x)1 + tan^2(x) = sec^2(x). Therefore,
sec2(θ2)tan(θ2)\frac{sec^2(\frac{\theta}{2})}{tan(\frac{\theta}{2})}
Now, rewrite in terms of sine and cosine:
1cos2(θ2)sin(θ2)cos(θ2)=1cos2(θ2)cos(θ2)sin(θ2)=1cos(θ2)sin(θ2)\frac{\frac{1}{cos^2(\frac{\theta}{2})}}{\frac{sin(\frac{\theta}{2})}{cos(\frac{\theta}{2})}} = \frac{1}{cos^2(\frac{\theta}{2})} \cdot \frac{cos(\frac{\theta}{2})}{sin(\frac{\theta}{2})} = \frac{1}{cos(\frac{\theta}{2})sin(\frac{\theta}{2})}
Multiply numerator and denominator by 2:
22sin(θ2)cos(θ2)\frac{2}{2sin(\frac{\theta}{2})cos(\frac{\theta}{2})}
Using the double angle formula sin(2x)=2sin(x)cos(x)sin(2x) = 2sin(x)cos(x), we have:
sin(θ)=2sin(θ2)cos(θ2)sin(\theta) = 2sin(\frac{\theta}{2})cos(\frac{\theta}{2})
Therefore,
2sin(θ)\frac{2}{sin(\theta)}
Since csc(θ)=1sin(θ)csc(\theta) = \frac{1}{sin(\theta)},
2csc(θ)2csc(\theta)
Thus, tan(θ2)+cot(θ2)=2csc(θ)tan(\frac{\theta}{2}) + cot(\frac{\theta}{2}) = 2csc(\theta).

3. Final Answer

tan(θ2)+cot(θ2)=2csc(θ)tan(\frac{\theta}{2}) + cot(\frac{\theta}{2}) = 2csc(\theta) is proven.

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