Can you solve this 5 radicals problem?

Algebra Level 5

x y 3 + y 2 9 y + 3 x = 30 x 30 y + 35 3 + 3 + y x \sqrt{x - y - 3} + \sqrt{y ^ {2} - 9y + 3x} = \sqrt[3] {30x - 30 y + 35} + \sqrt{3 + y - x}

If reals x x and y y satisfy the equation above and z = 2.4 x + 0.7 y 5 z = \sqrt[5]{2.4x +0.7y} , then find the sum of all possible values of z z .


The answer is 3.

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1 solution

Karim Fawaz
Jun 12, 2016

x y 3 + y 2 9 y + 3 x = \sqrt{x - y - 3} + \sqrt{y ^ {2} - 9y + 3x} = 30 x 30 y + 35 3 + 3 + y x \sqrt[3]{30x - 30 y + 35} + \sqrt{3 + y - x}

From the square root restrictions we have:

x - y - 3 >= 0 (#1)

y 2 9 y + 3 x > = 0 y ^ {2} - 9y + 3x >= 0 (#2)

3 + y - x >= 0 (#3)

From (1): x - y >= 3 (#4)

From (3): x - y <= 3 (#5)

From (4) and (5) : x - y = 3 i.e. y = x - 3 (#6)

Substituting y by (#6) in the original equation we get:

x x + 3 3 + ( x 3 ) 2 9 ( x 3 ) + 3 x = 30 x 30 x + 90 + 35 3 + 3 + x 3 x \sqrt{x - x + 3 - 3} + \sqrt{(x - 3) ^ {2} - 9(x - 3) + 3x} = \sqrt[3] {30x - 30x + 90 + 35} + \sqrt{3 + x - 3 - x}

0 + ( x 2 6 x + 9 9 x + 27 + 3 x ) = 125 3 + 0 \sqrt{0} + \sqrt{(x ^ {2} - 6x + 9 - 9x + 27 + 3x)} = \sqrt[3] {125} + \sqrt{0}

x 2 12 x + 36 = 5 \sqrt{x ^ {2} - 12x + 36} = 5

|x - 6| = 5

x - 6 = 5 or x - 6 = -5

if x - 6 = 5, then x = 11, y = 11 - 3 = 8, z = 2.4 ( 11 ) + 0.7 ( 8 ) 5 = 32 5 = 2 \sqrt[5] {2.4(11) + 0.7(8)} = \sqrt[5] {32} = 2

if x - 6 = -5, then x = 1, y = 1 - 3 = -2, z = 2.4 ( 1 ) + 0.7 ( 2 ) 5 = 1 5 = 1 \sqrt[5] {2.4(1) + 0.7(-2)} = \sqrt[5] {1} = 1

Total of all values of z = 1 + 2 = 3

A n s w e r = 3 Answer = \boxed{3}

Moderator note:

Interesting question. Nice way to slide in that x y = 3 x - y = 3 .

How was the z z function conceived? It has such a strange setup.

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