## Jamierox4ev3r one year ago Find the exact value of:

1. Jamierox4ev3r

$$\Large\cos (-\frac{5\pi}{4})$$

2. Jamierox4ev3r

the negative sign is throwing me off

3. zzr0ck3r

$$\cos(-x)=\cos(x)$$

4. Jamierox4ev3r

wait. so is it basically just saying $$\Large\cos (\frac{5\pi}{4})$$ o-o

5. zzr0ck3r

correct

6. matlee

$-\frac{ 1 }{ \sqrt{2} }$

7. zzr0ck3r

@matlee we all know how to use a calculator.

8. matlee

your smart , i hope you do

9. zzr0ck3r

read the rules, if you can

10. anonymous

11. anonymous

cos(-5pi/4) = cos(5pi/4) =cos(2pi -3pi/4) =cos(3pi/4) =cos(pi-pi/4) =-cospi/4 =-1/sqrt(2) Source: mathskey.com

12. Jamierox4ev3r

@matlee and @bradely , please don't provide direct answers, I don't care what the source is. So from what i see, $$\Large\cos (-\frac{5\pi}{4}) = -\frac{\sqrt{2}}{2}$$

13. Jamierox4ev3r

So what was the purpose of putting a negative sign in there? Just to throw of poor unsuspecting students? Or is there actually something I need to do about the said negative sign?

14. Jamierox4ev3r

@Nnesha @zzr0ck3r

15. Jamierox4ev3r

and @satellite73 thank you for the trig cheat sheet, lot of helpful things on there

16. Nnesha

cos is even function cos(-x)= cos(x) but careful sin and tan sin(-x)=-sin(X) odd tan(-x)=-tan(x)odd

17. Nnesha

18. Nnesha

so if cos (-x)= cos(x) then sec(-x)= ?

19. Jamierox4ev3r

then sec(-x) would be sec(x), since sec is just the inverse function of cos

20. Nnesha

so -cos(-x)= ?

21. Jamierox4ev3r

-cos(-x)= -cos(x)

22. Nnesha

yep! there is an identity to prove cos(-x)=cos(x) you'll learn n calc one i guess

23. Jamierox4ev3r

funnnn

24. Nnesha

ye!

25. Jamierox4ev3r

XD Thank you

26. Nnesha

np :=)

27. zzr0ck3r

cos gives you the $$x$$ value on the angle, |dw:1441072239511:dw| The bottom angle is the negative version of the top angle, but they both give the same x values.

28. zzr0ck3r

I hope that makes sense.