answer:
N(t)=2×[tex]1.7^{t}[/tex]
17
15
step by step explain:
before lesson (t=0), she knows 2 words.
after a week (t=1), she knows
2×(1+70%)=3.4 words
after one more week (t=2), she knows
3.4×(1+70%)=5.78 words
one more week later (t=3), she knows
5.78×(1+70%)=9.826 words
and so on ...
from pattern shown above, we know that she knows
2×[tex]1.7^{t}[/tex] words after t weeks
so N(t)=2×[tex]1.7^{t}[/tex]
after 4 weeks (t=4), she knows 2×[tex]1.7^{4}[/tex]=16.7042≈17 words
for learning 5000 words, she need:
2×[tex]1.7^{t}[/tex]=5000
[tex]1.7^{t}[/tex]=2500
t log(1.7)=log(2500)
t=[tex]\frac{log(2500)}{log(1.7)}[/tex]
=14.7448727
≈15 (round up)
