Followers 0

# trig help!

have a question which i just cant figure outt!!!

sinx=5cosx

i can get that tanx=5

and that tan-1x=78.69

now i have to solve on 0<x<10...how do i do that??

i know the answers are: 1.37, 4.515, 7.66...

thxxx

##### Share on other sites

Now that just made me realize that I MUST revise trig....

I wish I could help. If I had my math book, I would probably quickly refresh my memory and then try to solve but unfortunatley it's in school.

##### Share on other sites

is this SL? I don't wanna go doing HL stuff and get it wrong trig wasn't my specialty.. if it's SL I can probably do it...

I'm trying to do it now.. dunno if I can..

hey does the question say it wants exact figures? if not use ur graphing software and just do it on ur calc

##### Share on other sites

yes its SL...hehe im not a math genius, eh!?

Edited by beli16

##### Share on other sites

yea looks SL lol,

lsn "and that tan-1x=78.69" this part isn't very clear... do you mean tanx - 1x or tan(-x)? or did you mean tan inverse?

##### Share on other sites
yea looks SL lol,

lsn "and that tan-1x=78.69" this part isn't very clear... do you mean tanx - 1x or tan(-x)?

arctan..like tan to the -1

##### Share on other sites

oka simplest way? graph "tanx" then graph "5" on ur GDC, then find all point where they intersect and viola

##### Share on other sites

i did but on my gdc the tan equation is like really weird.....n also the teacher wants a mathematical procedure:s

##### Share on other sites

maybe u have it set as degrees instead of radians... (your calc) as for the mathematical thing.. first convert the degree value u have there into radians, and that would be the first one for the other two.. erm you have to figre out which quartile the angle is on (I think it's the 2nd quartile) and since this is tan then you'll have to subtract what you get with (pi). I don't really remember the quartile stuff really well since we could use GDC for paper one, if u have it in ur notes just review them (I kinda threw out all my math notes )

EDIT wait no! lol add (pi) to ur answer, don't subtract it, they're 1st and 3rd quartile not 2nd

##### Share on other sites

heheh its ok...so how do i transform 5 to radians....2pi=360...

##### Share on other sites

you don't transform the 5 to radians since 5 is not an angle! you transform the 78.69 into radians (divide it by 180 mutiply it to pi)

##### Share on other sites

of the 5 you're mentioning is in degrees then;

Edit: Oh whateva Lc came first. Didn't catch the 5 though...

##### Share on other sites

no GDC for paper one would've caused me to go down to a 6! lol I swear I can't do this stuff without my lovely TI-84 plus silver edition

##### Share on other sites

i knowww i can solve it with the calc noww!! thx LC n AFTERGLOW for all ur help!!!

ill ask my teacher n then ill tell u guys...if ur interested

lucky 2008s....no calculator for pp1!! im going to FAILLLLLL

##### Share on other sites

aww no you won't!

and I'm not really interested I hate trig!!

and yea ur teacher will probably make things much clearer I tried I'm not cut out to be a math helper

##### Share on other sites

Simultaneous equation!

My mind's not sufficiently with me at the moment, but as with intersecting 'lines' on the GDC graph, you can set up a simultaneous equation and solve.

##### Share on other sites
Simultaneous equation!

My mind's not sufficiently with me at the moment, but as with intersecting 'lines' on the GDC graph, you can set up a simultaneous equation and solve.

hmmm i dnt see how this can be solved using simultaneous equation... if you could please type it, wud be nice to know different methods.

now beli16, you said that tan-1x=78.69

bt i believe you ment t tan-1(5)=78.69 since you take tan^-1 for both sides to have that x=78.69, now that is your critical angle. We know that tan should be positive since 5 is positive, thus your angle must be either in the first quadrant or the third quadrant. If we are using degrees, then your answer would be: x = 78.69 and x = 258.69

now if we are using radians then we go back to beginning at tan-1(5) in radians that would be = 1.37 thus making your critical angle 1.37. Again tan is positive thus the angle lies in 1st and 3rd quadrants so for the first quadrant x = 1.37, for the 3rd quadrant x = pi+1.37 = 4.51

Now it is a matter of what your limits are, you say that you should solve for 0<x<10. Here i get kinda unsure of whether i solved wrng or whether the limits are incorrect since if the limits are correct then the question should be solved in radians, if solved in radians then you have two answers since both are above 0 and lower than 10. As for our degrees answers they wont apply and thats why you shoudl use radians. plz recheck the limits and let me know what the answr is...

my trig is kinda rusty we covered it first semester last year... better get back and revise

hope i helped

Edited by HMSChocolate

##### Share on other sites
have a question which i just cant figure outt!!!

sinx=5cosx

i can get that tanx=5

and that tan-1x=78.69

now i have to solve on 0<x<10...how do i do that??

i know the answers are: 1.37, 4.515, 7.66...

thxxx

because tan is positive in the first and third quadrant, we need to find the value of tan in the third quadrant. the value is : 1.37 + pi = 4.515

now, the limit for x is 0<x<10. that means the value may exceed 2pi. so, we can get the third value of x which is 1.37 + 2pi = 7.66.

note that we do not take 4.515 + 2pi = 10.798 as a value of x because it is out of the limit.

in the end, the values of x are 1.37, 4.515, 7.66

##### Share on other sites

because tan is positive in the first and third quadrant, we need to find the value of tan in the third quadrant. the value is : 1.37 + pi = 4.515

now, the limit for x is 0<x<10. that means the value may exceed 2pi. so, we can get the third value of x which is 1.37 + 2pi = 7.66.

note that we do not take 4.515 + 2pi = 10.798 as a value of x because it is out of the limit.

in the end, the values of x are 1.37, 4.515, 7.66

i dnt get it??? why did u add 1.37 to 2pi... simply on the basis that x can exceed 2 pi?

##### Share on other sites
i dnt get it??? why did u add 1.37 to 2pi... simply on the basis that x can exceed 2 pi?

Well, 2pi is 360 degree or one revolution. So, the value of x can be more than 360 degree/2pi. When I add 1.37 to 2pi means that in a new revolution there will be another value of x such that tanx=5. The "angle between the x-axis and line in the unit circle" (sorry I forgot the right term) is the same, 1.37. To get the angle, we add 2pi to 1.37.

sorry i can't explain well cause it's hard to tell math without showing things (diagram, work) in front of you. Besides, when explaining math, my english went bad.

##### Share on other sites
Well, 2pi is 360 degree or one revolution. So, the value of x can be more than 360 degree/2pi. When I add 1.37 to 2pi means that in a new revolution there will be another value of x such that tanx=5. The "angle between the x-axis and line in the unit circle" (sorry I forgot the right term) is the same, 1.37. To get the angle, we add 2pi to 1.37.

sorry i can't explain well cause it's hard to tell math without showing things (diagram, work) in front of you. Besides, when explaining math, my english went bad.

its ok i understood what you are saying, it slipped through my head the idea of more than one revolution

thanks