SOLUTION: x^4+(sqrt 76)x^2-76=0

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Question 753021: x^4+(sqrt 76)x^2-76=0
Answer by DrBeeee(684) About Me  (Show Source):
You can put this solution on YOUR website!
Giving you some tricky questions are they? I think we can handle it. It just a little complicated on-line.
First, to reduce the order of the polynomial, let
(1) y = x^2, then your equation reduces to
(2) y^2 + sqrt(76)*y -76 = 0
Now use the quadratic equation to find the roots of (2).
(3) y = -sqrt%2876%29%2F2+%2B-+%281%2F2%29%2Asqrt%2876+-4%2A1%2A%28-76%29%29 or
(4) y = -sqrt%2876%29%2F2+%2B-+%281%2F2%29%2Asqrt%2876+%2B+4%2A76%29 or
(5) y = -sqrt%2876%29%2F2+%2B-+%281%2F2%29%2Asqrt%2876%29%2A%28sqrt%285%29%29 or
(6) y = %28sqrt%2876%29%2F2%29%2A%28-1+%2B-+sqrt%285%29%29
Since y = x^2 we get
(7) x = +/-sqrt(y) and since y has two roots we will get four roots for x.
Taking the square root of (6) gives us two roots of x
(8) x1+=+%2Bsqrt%28%28sqrt%2876%29%2F2%29%2A%28-1%2Bsqrt%285%29%29%29 and
(9) x2+=+-sqrt%28%28sqrt%2876%29%2F2%29%2A%28-1%2Bsqrt%285%29%29%29
Likewise we can write the other two roots of x as
(10) x3+=+%2Bsqrt%28%28sqrt%2876%29%2F2%29%2A%28-1-sqrt%285%29%29%29 and
(11) x4+=+-sqrt%28%28sqrt%2876%29%2F2%29%2A%28-1-sqrt%285%29%29%29
We acn check these roots by substitution into your original equation in x. I'll use x of (8) to show that
(12) x%5E4+%2B+sqrt%2876%29%2Ax%5E2+-76+=+0
We have
(13) x%5E2+=+sqrt%2876%29%2F2%2A%28-1%2Bsqrt%285%29%29 and
(14) x%5E4+=+76%2F4%2A%28-1%2Bsqrt%285%29%29%5E2
Now put (13) and (14) into (12) and get
(15) or
(16) 76%2F4%2A%28-1%2Bsqrt%285%29%29%5E2+%2B+76%2F2%2A%28-1%2Bsqrt%285%29%29+-+76+=+0
Multiply (16) by 4/76 and get
(17) %28-1%2Bsqrt%285%29%29%5E2+%2B+2%2A%28-1%2Bsqrt%285%29%29+-+4+=+0 or
(18) %28-1%2Bsqrt%285%29%29%2A%28-1%2Bsqrt%285%29%2B2%29+-+4+=+0 or
(19) %28-1%2Bsqrt%285%29%29%2A%28%2B1%2Bsqrt%285%29%29+-+4+=+0 or
(20) -1%2Bsqrt%285%29%5E2+-+4+=+0 or
(21) -1+%2B+5+-+4+=+0 or
(22) 0 = 0 check
Answers: the roots of your quardic equation are given by (8)-(11)