[填空题]
The number of French words, N , that ar idg31 -tzjsh.e remembered by studadp/g 34z9.i3i9ec f njcl.tcents after the completion of a French language course decreases exponentially over time. This data can be m4cp 3t3i/.cd9za9. e nl ifgcjodelled 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 .
