If the largest real root of x 6 − 4 x 4 − 2 x 3 + 4 x 2 + 4 x + 1 = 0 can be expressed as c a + b , with a square-free and b and c are relatively prime, find ( a b c ) 2 + 1
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Notice this should be the square of a function in order to be solved by immediate means. This will be proved later. By the rational root theorem, we can see − 1 is a root. Now consider a number such that x 2 − x = 1 . By manipulation, we can see the roots of this and -1 would be added to a cubic that would be the square root. This can be done x 2 ( x 2 ) − x 2 ( x ) = x 2 , x 2 ( x + 1 ) − x ( x + 1 ) = x 2 , x 3 − x = x + 1 or x 3 − 2 x − 1 = 0 . If we square this, we get the desired expression. Now we see of the roots of x 2 − x = 1 , the golden ratio ( ( s q r t 5 +1)/2) is the largest. So the answer is 1 0 1 .
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Here was the method I used. Use synthetic division with -1 twice to get x 4 − 2 x 3 − x 2 + 2 x + 1 = 0 . Then divide by x 2 , and regroup the terms to get ( x 2 − 2 + x 2 1 ) + ( − 2 x + x 2 ) + 1 = 0 ⇒ ( x − x 1 ) 2 − 2 ( x − x 1 ) + 1 = 0 ⇒ ( x − x 1 − 1 ) 2 = 0 ⇒ x 2 − x − 1 = 0 . The larger root of this equation is 2 5 + 1 .