Maths Olympiad Prep

Library / /8 of 29

Algebra Difficulty 5.3 AIME, harder Prove it Croatia

Let aa, bb, cc be real numbers and let a0a \neq 0. Prove that at least one of the equations
asinx+bcosx+c=0, a \sin x + b \cos x + c = 0,
a y + b y + 2c = 0\text{a y + b y + 2c = 0}
has real solutions.

Solution

Let us consider the first equation:
asinx+bcosx+c=0. a \sin x + b \cos x + c = 0.
This can be rewritten as:
asinx+bcosx=c. a \sin x + b \cos x = -c.
Let R=a2+b2R = \sqrt{a^2 + b^2} and let φ\varphi be such that a=Rsinφa = R \sin \varphi, b=Rcosφb = R \cos \varphi.
Then:
asinx+bcosx=Rsinxsinφ+Rcosxcosφ=Rcos(xφ). a \sin x + b \cos x = R \sin x \sin \varphi + R \cos x \cos \varphi = R \cos(x - \varphi).
So the equation becomes:
Rcos(xφ)=c. R \cos(x - \varphi) = -c.
This equation has a real solution for xx if and only if cR|c| \leq R.

Suppose c>R|c| > R. Then the first equation has no real solution.

Now consider the second equation:
a y + b y + 2c = 0.\text{a y + b y + 2c = 0.}
Let t=tanyt = \tan y, tRt \in \mathbb{R}, t0t \neq 0.
Then y = t\text{y = t}, ctgy=1/t\ctg y = 1/t.
So the equation becomes:
at+bt+2c=0. a t + \frac{b}{t} + 2c = 0.
Multiply both sides by tt (for t0t \neq 0):
at2+2ct+b=0. a t^2 + 2c t + b = 0.
This is a quadratic equation in tt with discriminant:
Δ=(2c)24ab=4c24ab=4(c2ab). \Delta = (2c)^2 - 4a b = 4c^2 - 4a b = 4(c^2 - a b).
Since a0a \neq 0, this quadratic equation has real solutions if and only if c2ab0c^2 - a b \geq 0.

But recall that we are considering the case c>R=a2+b2|c| > R = \sqrt{a^2 + b^2}.
So c2>a2+b2c^2 > a^2 + b^2.
Therefore,
c2ab>a2+b2ab=(ab)20. c^2 - a b > a^2 + b^2 - a b = (a - b)^2 \geq 0.
So the discriminant is positive, and the quadratic equation has real solutions for t0t \neq 0.

Therefore, at least one of the two equations has real solutions.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.