Worry-able everywhere - 3

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 the irrational values of x x satisfying the above equation.


The answer is 6.066.

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1 solution

Yee-Lynn Lee
Aug 1, 2015

Our four cases and their discriminants (from previous problem(s)):

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 second and third equations have positive but non perfect square roots, so they will yield irrational solutions.

Using the quadratic formula on the second equation, b ± b 2 4 a c 2 \frac { -b\pm \sqrt { { b }^{ 2 }-4ac } }{ 2 } , gives us the solutions x = 1 ± 229 2 x=\frac { -1\pm \sqrt { 229 } }{ 2 } .

These solutions will make the base 0 0 , but we must check to see what they make the powers, 3 x 2 + 3 a n d 10 x { 3{ x }^{ 2 }+3\quad and\quad 10x } , because if the power(s) is negative, then it would equal 1 0 n \frac { 1 }{ { 0 }^{ n } } , which is undefined.

Substituting in x = 1 + 229 2 x=\frac { -1+\sqrt { 229 } }{ 2 } ,

3 ( 1 + 229 2 ) 2 + 3 24.199 3{ (\frac { -1+\sqrt { 229 } }{ 2 } ) }^{ 2 }+3\cong 24.199

10 ( 1 + 229 2 ) 70.664 10{ (\frac { -1+\sqrt { 229 } }{ 2 } ) }\cong 70.664

Therefore, this solution works.

Substituting in x = 1 229 2 x=\frac { -1-\sqrt { 229 } }{ 2 } ,

3 ( 1 229 2 ) 2 + 3 198.199 3{ (\frac { -1-\sqrt { 229 } }{ 2 } ) }^{ 2 }+3\cong 198.199

10 ( 1 + 229 2 ) 80.664 10{ (\frac { -1+\sqrt { 229 } }{ 2 } ) }\cong -80.664

The 10 x 10x power becomes negative, and therefore, this is not a valid solution.

Using the quadratic equation on the third equation, we get x = 1 ± 233 2 x=\frac { -1\pm \sqrt { 233 } }{ 2 }

Since 1 raised to any power, positive or negative, is 1, these solutions work.

Adding up the solutions, we get 1 + 233 2 + 1 233 2 + 1 + 229 2 \frac { -1+\sqrt { 233 } }{ 2 } { +\frac { -1-\sqrt { 233 } }{ 2 } +\frac { -1+\sqrt { 229 } }{ 2 } \cong } 6.066 \boxed {6.066}

Moderator note:

Good analysis.

Same comments about exposition apply.

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