[填空题]
The number of French words, N , that are remembefl( ))w 13nw, ffehgibred by studenthqdf6 2d7zj; )7aj lel5;gs:u 0jt(za kpnp8gs after the completion of a French language course decrea n70lfgj apgth;jpau d(ejkz)d:z68;5 q2l 7sses exponentially over time. This data can be modelled by the function
N(t)=a $\times b^{-t}+450$,
where a and b are positive constants, and t is the time in years since a student completed the French language course.
Immediately after completion, a student remembers 4200 French words.
1. Find the value of a .
After 4 years a student remembers only 1600 French words.
2. Find the value of b .
The number of French words a student remembers never decreases below a certain number of words, n . ≈
3. Write down the value of n .
