Squaring both sides of the original equation, we get a2=4+√5+a.
Subtracting 4 from both sides and squaring again gives (a2−4)2=5+a.
Expanding this out and subtracting 5 + a from both sides, we have a4−8a2−a+11=0.
Similar manipulations on the other equations give b4−8b2−b+11=0,c4−8c2+c+11=0,d4−8d2+d+11=0.
(Note the signs carefully.)
Let f(x)=x4−8x2−x+11. Then a and b are roots of f(x), while c and d are roots of x4−8x2+x+11, which is f(-x). It follows that -c and -d are roots of f(x). Since -c and -d are negative and hence distinct from a and b, we have four roots of f(x). Since f(x) has degree four, we know we have found all the roots.
By Vieta's formulas, the constant term of f(x) is the product of the roots. Thus,
abcd=(a)(b)(−c)(−d)=11.
