Worry-able everywhere - 2

Algebra Level 5

( x 2 + x 57 ) 3 x 2 + 3 = ( x 2 + x 57 ) 10 x \left( x^2+x-57 \right)^{3x^2+3}=\left( x^2+x-57 \right)^{10x} Find the sum (up to 3 decimal places) of all rational values of x x satisfying the above equation.


The answer is 10.333.

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1 solution

Yee-Lynn Lee
Aug 1, 2015

Looking back on our 4 cases and the discriminant of each from the solution to the first problem:

Case 1: 3 x 2 + 3 = 10 x 3{ x }^{ 2 }+3=10x

Since both sides have the same base, if the powers are equal, the two sides will equal each other.

Discriminant: 64 64

Case 2: x 2 + x 57 = 0 { x }^{ 2 }+x-57=0

Zero to any positive power will equal zero, so both sides will equal regardless of having different powers.

Discriminant: 229 229

Case 3: x 2 + x 57 = 1 { x }^{ 2 }+x-57=1

Similarly, 1 to any power will equal 1.

Discriminant: 233 233

Case 4: x 2 + x 57 = 1 { x }^{ 2 }+x-57=-1

As long as the powers on both sides are either both even or both odd, -1 to a power will equal 1 and -1, respectively.

Discriminant: 225 225

We see that the first case and the fourth case yield discriminants that are perfect squares, and will therefore yield rational solutions.

Factoring the first equation gives us

( 3 x 1 ) ( x 3 ) = 0 (3x-1)(x-3)=0

So, x = 3 , x = 1 3 x=3,x=\frac { 1 }{ 3 }

Substituting these values in the original problem, we see that they both work.

Factoring the fourth equation gives us

( x 7 ) ( x + 8 ) = 0 (x-7)(x+8)=0

So, x = 7 , x = 8 x=7,\quad x=-8

However, since this case sets the base as 1 -1 , we must check if these values make both 3 x 2 + 3 3{ x }^{ 2 }+3 and 10 x 10x even or both odd.

Substituting x = 7 x=7 , we get 3 ( 7 ) 2 + 3 = 150 , 10 ( 7 ) = 70 3{ (7) }^{ 2 }+3=150,\quad 10(7)=70

These are both even, so the end equation is 1 = 1 1=1

Substituting x = 8 x=-8 , we get 3 ( 8 ) 2 + 3 = 195 , 10 ( 8 ) = 80 3{ (-8) }^{ 2 }+3=195,\quad 10(-8)=-80

Since one is odd and one is even, the end result in 1 = 1 -1=1 , which is not true.

We disregard the solution x = 8 x=-8 , so adding up the other solutions gives us 3 + 1 3 + 7 = 10 1 3 , o r 10. 3 3+\frac { 1 }{ 3 } +7=10\frac { 1 }{ 3 } ,\quad or\quad 10.\overline { 3 }

Rounding that off gives our answer, 10.333 \boxed {10.333}

Moderator note:

Once again, being explicit in stating your cases will make it easier to understand and follow through your solution.

I'm glad that you checked the parity of exponents in the 4th case, and realized that one of them is not a solution.

Same solution

Dev Sharma - 5 years, 7 months ago

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