1. Ta có: AN=∣b−tct∣=t−bct
Vì t−bct≤4c⇒t≥34b
Lại có: AM=t≤4b⇒34b≤t≤4b
2. Ta có: 34b≤t≤4b⇒34≤bt≤4
Đặt x=bt
Ta có f(t)=2(t−b)ct2=2(bt−1).bcb2.(bt)2=2(x−1)cb.x2=bc.2(x−1)x2
Ta thấy: x2=x2+4−4≥4x−4=4(x−1)⇒f(t)≥2
Lại có: (x−34)(x−4)≤0⇔x2−316x+316≤0⇔x2≤316(x−1)⇒f(t)≤38