A weird system of equation

Geometry Level 5

x = y 2 1 16 + z 2 1 16 y = z 2 1 25 + x 2 1 25 z = x 2 1 36 + y 2 1 36 \begin{aligned} x&=&\sqrt{y^2-\dfrac{1}{16}}+\sqrt{z^2-\dfrac{1}{16}} \\ y&=&\sqrt{z^2-\dfrac{1}{25}}+\sqrt{x^2-\dfrac{1}{25}} \\ z&=&\sqrt{x^2-\dfrac{1}{36}}+\sqrt{y^2-\dfrac{1}{36}} \\ \end{aligned}

Given that x , y x, y and z z are real numbers that satisfy the system of equations above. If x + y + z = m n x+y+z=\dfrac{m}{\sqrt{n}} where m , n m, n are positive integers and n n is square-free, find the value of m + n m+n .

This problem is not original


The answer is 9.

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2 solutions

Parth Lohomi
Feb 2, 2015

Let X Y Z \triangle XYZ be a triangle with sides of length x , y x, y and z z , and suppose this triangle is acute (so all altitudes are on the interior of the triangle). Let the altitude to the side of length x x be of length h x h_x , and similarly for y y and z z . Then we have by two applications of the Pythagorean Theorem that x = y 2 h x 2 + z 2 h x 2 x = \sqrt{y^2 - h_x^2} + \sqrt{z^2 - h_x^2} . As a function of h x h_x , the RHS of this equation is strictly decreasing, so it takes each value in its range exactly once. Thus we must have that h x 2 = 1 16 h_x^2 = \dfrac1{16} and so h x = 1 4 h_x = \dfrac{1}4 and similarly h y = 1 5 h_y = \dfrac1{5} and h z = 1 6 h_z = \dfrac1{6} Since the area of the triangle must be the same no matter how we measure, x h x = y h y = z h z x\cdot h_x = y\cdot h_y = z \cdot h_z and so x 4 = y 5 = z 6 = 2 A \dfrac x4 = \dfrac y5 = \dfrac z6 = 2A and x = 8 A , y = 10 A x = 8A, y = 10A and z = 12 A z = 12A . The semiperimeter of the triangle is s = 8 A + 10 A + 12 A 2 = 15 A s = \dfrac{8A + 10A + 12A}{2} = 15A so by Heron's formula we have A = 15 A 7 A 5 A 3 A = 15 A 2 7 A = \sqrt{15A \cdot 7A \cdot 5A \cdot 3A} = 15A^2\sqrt{7} . Thus A = 1 15 7 A = \frac{1}{15\sqrt{7}} and x + y + z = 30 A = 2 7 x + y + z = 30A = \frac2{\sqrt{7}} and the answer is 2 + 7 = 9 2 + 7 = \boxed{9}

Perfect combination of Geometry and Algebra to solve a Number Theory problem. Nice job thumb-ups .

Trung Đặng Đoàn Đức - 6 years, 3 months ago

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Thanks @Trung Đặng Đoàn Đức

Parth Lohomi - 6 years, 3 months ago

Very nice problem!

Joel Tan - 6 years, 3 months ago

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Thanks @Joel Tan

Parth Lohomi - 6 years, 3 months ago

are u really 13???

Chirag Singapore - 6 years, 3 months ago

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Yes!! @Chirag Singapore

Parth Lohomi - 6 years, 3 months ago

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is this the first time you encountered a problem whose solution is related to triangles or you have done it before. And y do you have an IIt Bombay symbol ?

Chirag Singapore - 6 years, 3 months ago

to get the answer follow the steps given below:

take one radical to LHS and square both sides in the 3 equations and simplify

now leave these 3 equations and take other radical to LHS (which was left in previous step) and square both sides.

get relation b/w x,y and x,z ie x=+-0.8*y and x=+-0.67z

put all values in terms of x in 1st equation and get x,y&z.

x+y+z=2/7^0.5

answer=9

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