## onegirl Group Title 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. one year ago one year ago

1. onegirl Group Title

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. EulersEquation Group Title

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

3. onegirl Group Title

yes prenthesis

4. onegirl Group Title

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

5. EulersEquation Group Title

Your teacher specifically said to differentiate?

6. onegirl Group Title

yes

7. onegirl Group Title

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

8. onegirl Group Title

@EulersEquation so do you know how?

9. onegirl Group Title

@Numb3r1 can u help?

10. EulersEquation Group Title

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. onegirl Group Title

ok

12. EulersEquation Group Title

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. onegirl Group Title

okay

14. onegirl Group Title

@waterineyes

15. waterineyes Group Title

Just want to take derivative of that??

16. onegirl Group Title

yes Find f' and f'(0)

17. waterineyes Group Title

It will come out to be negative..

18. onegirl Group Title

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

19. waterineyes Group Title

Wait, let we do it first..

20. waterineyes Group Title

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

21. waterineyes Group Title

It will become positive now.. :)

22. onegirl Group Title

okay

23. EulersEquation Group Title

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

24. waterineyes Group Title

Am I right in this?

25. EulersEquation Group Title

Ah! I forgot to square the denominator!

26. EulersEquation Group Title

That looks good.

27. onegirl Group Title

okay thanks!

28. waterineyes Group Title

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

29. onegirl Group Title

okay

30. waterineyes Group Title

@onegirl can you go for that?

31. onegirl Group Title

just plug in 0 and solve right?

32. waterineyes Group Title

Yep, just simple calculations with calculator..

33. onegirl Group Title

okay yes i can

34. waterineyes Group Title

Good.. Keep it up..

35. onegirl Group Title

i got 0 after i solved