# Leo Giugiuc's Trigonometric Lemma

Leo Giugiuc has posted the following statement and its proof at the CutTheKnotMath facebook page:

Let $x$ and $y$ be real numbers such that $0\lt x\lt y\lt\pi.$

- If $x+y\lt\pi$ then $\sin x\lt\sin y.$
- If $x+y\gt\pi$ then $\sin x\gt\sin y.$

### Proof 1 of 1.

In this case necessarily $\displaystyle x\lt\frac{\pi}{2}$ because, otherwise, $\displaystyle y\gt\frac{\pi}{2},$ contradicting the premise. By the hypothesis, $x\lt y$ and also $x\lt\pi -y.$ But, one of the two quantities - $y$ and $\pi -y$ - is bound to be in $\displaystyle \left[0,\frac{\pi}{2}\right]$ and, since *sine* is strictly increasing on $\displaystyle \left[0,\frac{\pi}{2}\right],$ either $\sin x\lt\sin y$ or $\sin x\lt\sin (\pi -y),$ which proves #1 because $\sin y=\sin (\pi -y).$

### Proof 2 of 1.

Consider $\Delta ABC,$ with angles $x$ at $A,$ $y$ at $B,$ and $\pi -x-y$ at $C.$ Note that $BC\lt AC,$ for $x\lt y.$ By the Law of Sines, $\displaystyle\frac{AC}{BC}=\frac{\sin x}{\sin y},$ implying $\sin x\gt\sin y.$

### Proof of 2.

In this case, $0\lt\pi y\lt\pi -x$ and $(\pi -y)+(\pi -x)\lt 2\pi -\pi =\pi.$ Hence, by the first part, with $\pi -y$ for $x$ and $\pi -x$ for $y,$ we conclude that $\sin (\pi -y)\lt\sin (\pi -x),$ or, equivalently, $\sin y\lt\sin x.$

### Trigonometry

- What Is Trigonometry?
- Addition and Subtraction Formulas for Sine and Cosine
- The Law of Cosines (Cosine Rule)
- Cosine of 36 degrees
- Tangent of 22.5
^{o}- Proof Wthout Words - Sine and Cosine of 15 Degrees Angle
- Sine, Cosine, and Ptolemy's Theorem
- arctan(1) + arctan(2) + arctan(3) = π
- Trigonometry by Watching
- arctan(1/2) + arctan(1/3) = arctan(1)
- Morley's Miracle
- Napoleon's Theorem
- A Trigonometric Solution to a Difficult Sangaku Problem
- Trigonometric Form of Complex Numbers
- Derivatives of Sine and Cosine
- ΔABC is right iff sin²A + sin²B + sin²C = 2
- Advanced Identities
- Hunting Right Angles
- Point on Bisector in Right Angle
- Trigonometric Identities with Arctangents
- The Concurrency of the Altitudes in a Triangle - Trigonometric Proof
- Butterfly Trigonometry
- Binet's Formula with Cosines
- Another Face and Proof of a Trigonometric Identity
- cos/sin inequality
- On the Intersection of kx and |sin(x)|
- Cevians And Semicircles
- Double and Half Angle Formulas
- A Nice Trig Formula
- Another Golden Ratio in Semicircle
- Leo Giugiuc's Trigonometric Lemma
- Another Property of Points on Incircle
- Much from Little
- The Law of Cosines and the Law of Sines Are Equivalent
- Wonderful Trigonometry In Equilateral Triangle
- A Trigonometric Observation in Right Triangle
- A Quick Proof of cos(pi/7)cos(2.pi/7)cos(3.pi/7)=1/8

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