>>350 >>353

(a^2 -b)(c^2 -d) = (7/6 -f/2)^2 - (20/27)^2/(7/6 +f/2)
 = (1/3) [(13+2√11)^{1/3} - (13-2√11)^{1/3}]^2
 = -0.237618966426144
 < 0,