## onegirl Group Title Find the derivative of f(x) = cos x, a = pi/2 one year ago one year ago

1. onegirl Group Title

@zepdrix i ddi it but can u check my answer?

2. zepdrix Group Title

sure :O what are you suppose to do? Just find the derivative then plug $$\large a$$ in? Or does this have something to do with approximation?

3. onegirl Group Title

okay my teacher said there is an equation, so here is what i did f(x) = cos (x) f'(x) = -sin(x) x = a = pi/2 so f'(pi/2) = -sin (pi/2) = -1

4. zepdrix Group Title

Your teacher said there is an equation? :) That's not much detail lolol Your derivative looks correct at least.

5. onegirl Group Title

yea she said try again there is an equation

6. onegirl Group Title

let me show you how i wrote it first okay?

7. onegirl Group Title

i mean the answer i wrote first

8. zepdrix Group Title

k

9. onegirl Group Title

d/dx ( cos (x)) = -sin(x) -sin (pi/2) = -1, so the slope is -1, y = mx + b --> y = =1x + b cos (pi/2) = 0 so the coordinates pare are (pi/2, 0) so y = 0x + b, 0(pi/2) + b equals to 0, 0 = y = 0x + 0 = y = 0 for my final answer.

10. zepdrix Group Title

Oh so like the previous problems, you're looking for an equation of the tangent line?

11. onegirl Group Title

yes sorry i made a mistake

12. onegirl Group Title

yes yes the equation of the tangent line

13. zepdrix Group Title

$\large y=-x+b$ From here you plugged in your point $$\large \left(\pi/2,\;0\right)$$. $\large 0=-\pi/2+b \qquad \qquad \rightarrow \qquad \qquad b=\pi/2$ I think you plugged your $$\large 0$$ into the wrong spot.

14. onegirl Group Title

ohhh okay

15. onegirl Group Title

so instead of 0 = 0(pi/2) + b what would i write

16. zepdrix Group Title

$\huge (\color{royalblue}{\pi/2},\;\color{orangered}{0}) \qquad = \qquad (\color{royalblue}{x},\;\color{orangered}{y})$ Plug them into here, match up the colors.$\huge \color{orangered}{y}=(-1)\color{royalblue}{x}+b$ I'm not sure why you're plugging a $$\large 0$$ in for your -1....

17. onegirl Group Title

okay

18. onegirl Group Title

so 0 = (-1)(pi/2) + b ?

19. zepdrix Group Title

yes

20. onegirl Group Title

okay

21. onegirl Group Title

thx

22. zepdrix Group Title

np