[填空题]
The number of French wordsj; kuxl8n*2dhtt c/j., N, that are remembered by students wyptxo vh3c*p u3vxtz8 +7d1:after the completion of a French language course decreases uyhw3cp7tx+tod1zp3v :vx* 8exponentially 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. 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 =