[填空题]
The number of French words, N , that are rem8,fea0:rq8sh4* 908yjv ig qu yb hibyembered by students afteq+.ljq0 uid bm1ih e87xlf,;rr the completion of a French language course decreases exponentiaf ,i1ehl7+bxd 0i8 ujml.rq;q lly 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 .
