>>821
すまん
間違えた
(t, f(t))における接線は
y = 3(t^2-1)(x-t) + f(t)

0 = 3(t^2 -1)(s-t) + f(t)
3(t^2-1)(s-α) +f(t) -3(t^2-1)(t-α) =0

2(t^2-1)(t-α) < f(t) を示す

g(t) = f(t)-2(t^2-1)(t-α)
g(α) = 0
g(3) = 13- 16(3-α) = -35+16α

35/16 < 36/16 = 9/4
f(9/4) = -23/64 < 0だから、 α > 36/16 > 35/16となり
g(3) > 0

g''(t) = 4a-6t < 0
なので、g(t) > 0