Find the sum (up to 3 decimal places) of all rational values of satisfying the above equation.
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Looking back on our 4 cases and the discriminant of each from the solution to the first problem:
Case 1: 3 x 2 + 3 = 1 0 x
Since both sides have the same base, if the powers are equal, the two sides will equal each other.
Discriminant: 6 4
Case 2: x 2 + x − 5 7 = 0
Zero to any positive power will equal zero, so both sides will equal regardless of having different powers.
Discriminant: 2 2 9
Case 3: x 2 + x − 5 7 = 1
Similarly, 1 to any power will equal 1.
Discriminant: 2 3 3
Case 4: x 2 + x − 5 7 = − 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: 2 2 5
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
So, x = 3 , x = 3 1
Substituting these values in the original problem, we see that they both work.
Factoring the fourth equation gives us
( x − 7 ) ( x + 8 ) = 0
So, x = 7 , x = − 8
However, since this case sets the base as − 1 , we must check if these values make both 3 x 2 + 3 and 1 0 x even or both odd.
Substituting x = 7 , we get 3 ( 7 ) 2 + 3 = 1 5 0 , 1 0 ( 7 ) = 7 0
These are both even, so the end equation is 1 = 1
Substituting x = − 8 , we get 3 ( − 8 ) 2 + 3 = 1 9 5 , 1 0 ( − 8 ) = − 8 0
Since one is odd and one is even, the end result in − 1 = 1 , which is not true.
We disregard the solution x = − 8 , so adding up the other solutions gives us 3 + 3 1 + 7 = 1 0 3 1 , o r 1 0 . 3
Rounding that off gives our answer, 1 0 . 3 3 3