Given : f(x)=4x4−32x2+734(x2−4)
Dividing numerator by denominator, f(x) reduced to :
f(x)=(x2−4)+94(x2−4)
and (x2−4)>0 for |x|>2,
using A.M.≥G.M.:
⇒12[(x2−4)+94(x2−4)]≥√(x2−4)⋅94(x2−4)⇒(x2−4)+94(x2−4)≥2×32⇒(x2−4)+94(x2−4)≥3
So, minimum value of f(x) is 3.