## anonymous 3 years ago A person scores f(x) = 90/1 + 4e^-0.4t points on a test after t hours of studying. What does the person score without studying at all? Compute f’(0) and estimate how many points 1 hours of studying will add to the score.

1. anonymous

This is what I did but my teacher said it was wrong she said ( Find f' and f'(0).) F(x) = 90/ 1 + 4e^-0.4t F(0) = 90/ 1 + 4e^-0.4(0) = 18 F(1) = 90/1 + 4e^-0.4(1) = 24.45 So comparing the scote at f(1) and f(0) where at f(1) studying for 1 hour and f(0) studying none. F(1) - f(0)/24.45 – 18 = 6.45 points

2. anonymous

Should this have parenteses or not? Is this F(x) = 90/ (1 + 4e^-0.4t )?

3. anonymous

yes prenthesis

4. anonymous

like this $\frac{ 9 }{ 1 + 4e ^{-0.4t}}$

5. anonymous

Your teacher specifically said to differentiate?

6. anonymous

yes

7. anonymous

she said to Find f' and f'(0)

8. anonymous

@EulersEquation so do you know how?

9. anonymous

@Numb3r1 can u help?

10. anonymous

Yes. I need to review this. First glance is to use the quotient rule. Do you know how to use the quotient rule? Someone else may want to take a look. I am in the middle of helping someone else. When I finish I'll get on it.

11. anonymous

ok

12. anonymous

Ha! I am terrible at arithmetic. On first run I get a negative number for f'(0). You cannot make a negative score on a test! Unless the teacher doesn't like you. I am rusty on this. I'll work on it some more. In the meantime, bump the problem and hopefully someone else will tackle this.

13. anonymous

okay

14. anonymous

@waterineyes

15. anonymous

Just want to take derivative of that??

16. anonymous

yes Find f' and f'(0)

17. anonymous

It will come out to be negative..

18. anonymous

what will come out? so what i did above is wrong or?

19. anonymous

Wait, let we do it first..

20. anonymous

$\frac{ 90 }{ 1 + 4e ^{-0.4t}} = \frac{ -(90)(-1.6 \cdot e^{-0.4t}) }{ (1 + 4e ^{-0.4t})^2}$

21. anonymous

It will become positive now.. :)

22. anonymous

okay

23. anonymous

Yeah. I don't see why it has to be differentiated -- not according to the question.

24. anonymous

Am I right in this?

25. anonymous

Ah! I forgot to square the denominator!

26. anonymous

That looks good.

27. anonymous

okay thanks!

28. anonymous

So you can now find f'(0) by putting 0 as value of t..

29. anonymous

okay

30. anonymous

@onegirl can you go for that?

31. anonymous

just plug in 0 and solve right?

32. anonymous

Yep, just simple calculations with calculator..

33. anonymous

okay yes i can

34. anonymous

Good.. Keep it up..

35. anonymous

i got 0 after i solved