f(x) has extreme points at −1,0,1
⇒f′(x)=0 at x=–1,0,1
⇒f′(x)=a(x2−1)x∵f(x) is a polynomial
⇒f(x)=∫a(x3−x)dx=a(4x4−2x2)+C
Now the equation,
f(x)=f(0)
⇒a(4x4−2x2)+C=C
⇒4x4−2x2=0⇒x2(x2−2)=0
⇒x=0,±2
⇒S=0,2,−2.
Hence, the set contains two irrational and one rational number.