[填空题]
The number of French words, N, that are remembere rwjrt xs),ks*w,l,m( d by s7y*de8q y g2u-ai16bylg;0 5dhog pv rtudents after the completion of a French language course decreases exponentially ovedg5hya gqy - vl78reb;6o d0gi2*y1u pr 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. a =
After 4 years a student remembers only 1600 French words.
2.Find the value of b. 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. n =