University level Geometry question
Show that for any triangle ABC, the following inequality is true
a^2 + b^2 + c^2 > (3)^1/2 max [ | a^2 - b^2 |, | b^2 - c^2 |, | c^2 - a^2 | ]
Where a,b,c are the sides of the triangle in the usual notation.
(I tried using Law of cosine to solve it but I wasn't able to simplify the equations using that, I think inequality can be used to solve this question but my geometry is rusty )
1 Answer
Without loss of generality we may assume $a \geq b \geq c$. Then
\[\max \{ | a^2 - b^2 |, | b^2 - c^2 |, | c^2 - a^2 | \}=a^2-c^2.\]
It is enough to show that
\[a^2+b^2+c^2 = \sqrt{3}(a^2-c^2) (*).\]
By law of cosines we have
$$a^2+b^2+c^2 \geq a^2+(a^2+c^2-2ac \cos \theta)+c^2$$
\[\geq a^2+(a^2+c^2-2ac )+c^2=2(a^2+c^2-ac)\]
\[\geq \sqrt{3}(a^2-c^2)+(2-\sqrt{3})a^2+(2+\sqrt{3})c^2-2ac\]
\[\geq \sqrt{3}(a^2-c^2)+(\sqrt{(2-\sqrt{3})}a-\sqrt{(2+\sqrt{3})}c)^2\]
\[\geq \sqrt{3}(a^2-c^2).\]
Hence
\[a^2+b^2+c^2 \geq \sqrt{3}(a^2-c^2).\]
![Paul F](https://matchmaticians.com/storage/user/100013/thumb/matchmaticians-2in34k-file-1-avatar-512.jpg)
-
Thank you so much for helping me out with the solution Paul:)
- 1 Answer
- 945 views
- Pro Bono
Related Questions
- Find the volume of the solid obtained by rotating $y=x^2$ about y-axis, between $x=1$ and $x=2$, using the shell method.
- Thickness of Multiple Spherical Shells
- Geometry without using trigonometry
- Determine formula to calculate the radii of a unique ellipsoid from coordinates of non-coplanar locii on its surface, and without knowing its center or rotation angles.
- Why if $\frac{opp}{adj} =x$, then $x \times hyp =$ The length of a line perpendicular to the hypotenuse with the same height.
- How to recalculate 2D polygon side lengths when tilt is applied in 3D space?
- How can I find steering angle if the only information I have is arc length, vehicle length, and how far my vehicle is off course?
- Calculating aspect ratio limits of rotated rectangle within a rectangle