#### C. Do you want a date?

##### 解法

1. $a_2-a_1, (a_3-a_1)\cdot2, (a_4-a_1) \cdot 2^2, (a_5-a_1) \cdot 2^3$
2. $a_3-a_2, (a_4-a_2)\cdot2, (a_5-a_2)\cdot2^2$
3. $a_4-a_3, (a_5-a_3)\cdot2$
4. $a_5-a_4$

$F(a) =15a_5+6a_4-6a_4-15a_1\\=(1-16)a_5+(8-2)a_4+(4-4)a_3+(2-8)a_4+(1-15)a_1$