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Difference between revisions of "1966 IMO Problems/Problem 2"
Line 34: Line 34:
 
<math>(a + b) \tan \frac{\alpha + \beta}{2} = a \tan \alpha + b \tan \beta</math>.
 
<math>(a + b) \tan \frac{\alpha + \beta}{2} = a \tan \alpha + b \tan \beta</math>.
  
−
From <math>\alpha < \beta</math> it follows that <math>a < b</math>.  So,
+
From <math>\alpha < \frac{\pi}{2} \le \beta</math> it follows that <math>a < b</math>.  So,
  
 
<math>b \tan \beta = (a + b) \tan \frac{\alpha + \beta}{2} - a \tan \alpha >
 
<math>b \tan \beta = (a + b) \tan \frac{\alpha + \beta}{2} - a \tan \alpha >
Line 45: Line 45:
 
{1 - \tan^2 \frac{\alpha}{2}} > 0</math>
 
{1 - \tan^2 \frac{\alpha}{2}} > 0</math>
  
−
because the numerator is <math>> 0</math> and the denominator is also <math>> 0</math>
+
because the numerator is <math>> 0</math> (because <math>x^2 + x + 1 > 0</math>), and the
−
(because <math>\alpha < \frac{\pi}{2}</math> so <math>\tan \frac{\alpha}{2} < 1</math>).
+
denominator is also <math>> 0</math> (because <math>\alpha < \frac{\pi}{2}</math> so
 +
<math>\tan \frac{\alpha}{2} < 1</math>).
  
 
It follows that <math>\tan \beta > 0</math>, so it can not be that
 
It follows that <math>\tan \beta > 0</math>, so it can not be that
Line 55: Line 56:
 
implies <math>\alpha = \beta</math>.
 
implies <math>\alpha = \beta</math>.
  
 +
Replace <math>a = \sin \alpha \cdot 2R</math> and <math>b = \sin \beta \cdot 2R</math>
 +
(in fact, we don't care that <math>R</math> is the radius of the circumscribed
 +
circle), and simplify by <math>2R</math>.  We get
  
 +
<math>(\sin \alpha + \sin \beta) \tan \frac{\alpha + \beta}{2} =
 +
\sin \alpha \tan \alpha + \sin \beta \tan \beta</math>.
  
 +
This becomes
  
 +
<math>(\sin \frac{\alpha + \beta}{2} \tan \frac{\alpha + \beta}{2}) \cdot
 +
\cos \frac{\alpha - \beta}{2} =
 +
\frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta)</math>
  
 +
We will show that the function <math>f(x) = \tan x \sin x</math> is convex on
 +
the interval <math>(0, \frac{\pi}{2})</math>.  Indeed, the first derivative is
 +
<math>f'(x) = \frac{\sin x}{\cos^2 x} + \sin x</math>, and the second derivative
 +
is <math>f''(x) = \frac{\cos^4 x - \cos ^2 x + 2}{\cos^3 x}</math>.
  
 +
We have <math>f''(x) > 0</math> on <math>(0, \frac{\pi}{2})</math> since the numerator
 +
is <math>> 0</math> because <math>Y^2 - Y + 1 >0</math>, and the denominator is <math>> 0</math> on
 +
the interval <math>(0, \frac{\pi}{2})</math>.  It follows that
 +
<math>f(x) = \tan x \sin x</math> is convex on the interval <math>(0, \frac{\pi}{2})</math>.
 +
 +
Using the convexity we have
 +
<math>f \left( \frac{x + y}{2} \right) \le \frac{1}{2} (f(x) + f(y))</math>.  In
 +
our case, we have
 +
 +
<math>\frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta) =
 +
(\sin \frac{\alpha + \beta}{2} \tan \frac{\alpha + \beta}{2}) \cdot
 +
\cos \frac{\alpha - \beta}{2} \le
 +
\frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta) \cdot
 +
\cos \frac{\alpha - \beta}{2}</math>.
 +
 +
We can simplify by <math>\sin \alpha \tan \alpha + \sin \beta \tan \beta</math>
 +
because it is positive (because both <math>\alpha, \beta</math> are acute!),
 +
and we get <math>1 \le \cos \frac{\alpha - \beta}{2}</math>.  This is possible
 +
only when <math>\cos \frac{\alpha - \beta}{2} = 1</math>, i.e. <math>\alpha = \beta</math>.
  
 
(Solution by pf02, September 2024)
 
(Solution by pf02, September 2024)
−
 
−
TO BE CONTINUED.  SAVING MID WAY SO I DON'T LOOSE WORK DONE SO FAR.
 
  
  
 
==See Also==
 
==See Also==
 
{{IMO box|year=1966|num-b=1|num-a=3}}
 
{{IMO box|year=1966|num-b=1|num-a=3}}

Revision as of 02:22, 29 September 2024

Let $a$, $b$, and $c$ be the lengths of the sides of a triangle, and $\alpha,\beta,\gamma$ respectively, the angles opposite these sides. Prove that if

\[a+b=\tan{\frac{\gamma}{2}}(a\tan{\alpha}+b\tan{\beta}),\]

the triangle is isosceles.


Solution

We'll prove that the triangle is isosceles with $a=b$. We'll prove that $a=b$. Assume by way of contradiction WLOG that $a>b$. First notice that as $\gamma = \pi -\alpha-\beta$ then and the identity $\tan\left(\frac \pi 2 - x \right)=\cot x$ our equation becomes: \[a+b=\cot \frac{\alpha +\beta}{2}\left(a\tan \alpha + b\tan \beta \right)\]\[\iff a\tan\frac{\alpha +\beta}{2}+b\tan \frac{\alpha +\beta}{2}=a\tan \alpha + b\tan \beta\] \[\iff a\left(\tan \alpha -\tan \frac{\alpha +\beta}{2}\right)+b\left(\tan \beta -\tan \frac{\alpha +\beta}{2} \right)=0\] Using the identity $\tan (A-B)=\frac {\tan A-\tan B}{1+\tan A\tan B}$ $\iff \tan A-\tan B=\tan(A-B)(1+\tan A\tan B)$ and inserting this into the above equation we get: \[\iff a\tan \frac{\alpha -\beta}{2}\left(1+\tan \alpha \tan \frac{\alpha +\beta}{2}\right)+b\tan \frac{\beta -\alpha}{2}\left(1+\tan \beta \tan \frac{\alpha +\beta}{2} \right)=0\] \[\underbrace{\iff}_{\tan -A=-\tan A}a\tan \frac{\alpha -\beta}{2}\left(1+\tan \alpha \tan \frac{\alpha +\beta}{2}\right)-b\tan \frac{\alpha -\beta}{2}\left(1+\tan \beta \tan \frac{\alpha +\beta}{2} \right)=0\] \[\iff \tan \frac{\alpha -\beta}{2}\left(a-b+\tan \frac{\alpha +\beta}{2}(a\tan\alpha -b\tan \beta) \right)=0\] Now, since $a>b$ and the definitions of $a,b,\alpha,\beta$ being part of the definition of a triangle, $\alpha >\beta$. Now, $\pi >\alpha -\beta >0$ (as $\alpha+\beta +\gamma = \pi$ and the angles are positive), $\tan \frac{\alpha -\beta}{2}\neq 0$, and furthermore, $\tan \frac{\alpha+\beta}{2}>0$. By all the above, \[\left(a-b+\tan \frac{\alpha +\beta}{2}(a\tan\alpha -b\tan \beta) \right)>0\] Which contradicts our assumption, thus $a\leq b$. By the symmetry of the condition, using the same arguments, $a\geq b$. Hence $a=b$.


Solution 2

First, we'll prove that both $\alpha$ and $\beta$ are acute. At least one of them has to be acute because these are angles of a triangle. We can assume that $\alpha$ is acute. We want to show that $\beta$ is acute as well. For a proof by contradiction, assume $\beta \ge \frac{\pi}{2}$.

From the hypothesis, it follows that $(a + b) \tan \frac{\alpha + \beta}{2} = a \tan \alpha + b \tan \beta$.

From $\alpha < \frac{\pi}{2} \le \beta$ it follows that $a < b$. So,

$b \tan \beta = (a + b) \tan \frac{\alpha + \beta}{2} - a \tan \alpha > 2a \tan \frac{\alpha + \beta}{2} - a \tan \alpha \ge a (2 \tan \left( \frac{\alpha}{2} + \frac{\pi}{4} \right) - \tan \alpha) =$

$2a \left( \frac{\tan \frac{\alpha}{2} + 1}{1 - \tan \frac{\alpha}{2}} - \frac{\tan \frac{\alpha}{2}}{1 - \tan^2 \frac{\alpha}{2}} \right) = 2a \cdot \frac{\tan^2 \frac{\alpha}{2} + \tan \frac{\alpha}{2} + 1} {1 - \tan^2 \frac{\alpha}{2}} > 0$

because the numerator is $> 0$ (because $x^2 + x + 1 > 0$), and the denominator is also $> 0$ (because $\alpha < \frac{\pi}{2}$ so $\tan \frac{\alpha}{2} < 1$).

It follows that $\tan \beta > 0$, so it can not be that $\beta \ge \frac{\pi}{2}$.

Now, we will prove that $(a + b) \tan \frac{\alpha + \beta}{2} = a \tan \alpha + b \tan \beta$ implies $\alpha = \beta$.

Replace $a = \sin \alpha \cdot 2R$ and $b = \sin \beta \cdot 2R$ (in fact, we don't care that $R$ is the radius of the circumscribed circle), and simplify by $2R$. We get

$(\sin \alpha + \sin \beta) \tan \frac{\alpha + \beta}{2} = \sin \alpha \tan \alpha + \sin \beta \tan \beta$.

This becomes

$(\sin \frac{\alpha + \beta}{2} \tan \frac{\alpha + \beta}{2}) \cdot \cos \frac{\alpha - \beta}{2} = \frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta)$

We will show that the function $f(x) = \tan x \sin x$ is convex on the interval $(0, \frac{\pi}{2})$. Indeed, the first derivative is $f'(x) = \frac{\sin x}{\cos^2 x} + \sin x$, and the second derivative is $f''(x) = \frac{\cos^4 x - \cos ^2 x + 2}{\cos^3 x}$.

We have $f''(x) > 0$ on $(0, \frac{\pi}{2})$ since the numerator is $> 0$ because $Y^2 - Y + 1 >0$, and the denominator is $> 0$ on the interval $(0, \frac{\pi}{2})$. It follows that $f(x) = \tan x \sin x$ is convex on the interval $(0, \frac{\pi}{2})$.

Using the convexity we have $f \left( \frac{x + y}{2} \right) \le \frac{1}{2} (f(x) + f(y))$. In our case, we have

$\frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta) = (\sin \frac{\alpha + \beta}{2} \tan \frac{\alpha + \beta}{2}) \cdot \cos \frac{\alpha - \beta}{2} \le \frac{1}{2}(\sin \alpha \tan \alpha + \sin \beta \tan \beta) \cdot \cos \frac{\alpha - \beta}{2}$.

We can simplify by $\sin \alpha \tan \alpha + \sin \beta \tan \beta$ because it is positive (because both $\alpha, \beta$ are acute!), and we get $1 \le \cos \frac{\alpha - \beta}{2}$. This is possible only when $\cos \frac{\alpha - \beta}{2} = 1$, i.e. $\alpha = \beta$.

(Solution by pf02, September 2024)


See Also

1966 IMO (Problems) • Resources
Preceded by
Problem 1 		1 • 2 • 3 • 4 • 5 • 6 Followed by
Problem 3
All IMO Problems and Solutions
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