The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Trig question c2

Solve for theta in the interval 0 < theta < 360
3sin^2theta+sintheta
Factorising this gives sintheta(3sintheta + 1)
sintheta = 0 or sintheta =-1/3

solutions for sin theta is the same as a sin graph , but how do i find the solutions of sintheta -1/3?
Original post by Think People
Solve for theta in the interval 0 < theta < 360
3sin^2theta+sintheta
Factorising this gives sintheta(3sintheta + 1)
sintheta = 0 or sintheta =-1/3

solutions for sin theta is the same as a sin graph , but how do i find the solutions of sintheta -1/3?


Do the inverse of sin (-1/3) to get a value for theta. Then use a CAST diagram or graphical methods to find the solutions.

Watch this if you don't know how to use a CAST diagram:
https://www.youtube.com/watch?v=zGCnBgyM9kQ
(edited 8 years ago)
Original post by Think People
Solve for theta in the interval 0 < theta < 360
3sin^2theta+sintheta
Factorising this gives sintheta(3sintheta + 1)
sintheta = 0 or sintheta =-1/3

solutions for sin theta is the same as a sin graph , but how do i find the solutions of sintheta -1/3?


You do sine inverse to get the values of theta and draw a ASTC quadrant and see what values are valid for your range or you could just draw the sine graph and choose the valise which correspond to sin theta which are within your range :smile:
Original post by hellomynameisr
Do the inverse of sin (-1/3) to get a value for theta. Then use a CAST diagram or graphical methods to find the solutions.

Watch this if you don't know how to use a CAST diagram:
https://www.youtube.com/watch?v=zGCnBgyM9kQ


I tend to not use the CAST diagram method.
I draw the sin graph out then use symmetry to find the other points.For this type of question do you have any idea on how to do it without the cast method ? :frown:
Original post by Shiv Loves Maths
You do sine inverse to get the values of theta and draw a ASTC quadrant and see what values are valid for your range or you could just draw the sine graph and choose the valise which correspond to sin theta which are within your range :smile:


Inverse sin -1/3 gives -19.47... , where do i go from here
Original post by Think People
I tend to not use the CAST diagram method.
I draw the sin graph out then use symmetry to find the other points.For this type of question do you have any idea on how to do it without the cast method ? :frown:


so you draw the CAST diagram and then label the quadrants: q1 A, q2 S, q3 T, q4 C

since it is -1/3 and sin, you draw lines in q3 and 4

Then you can use the value of sin inverse (-1/3) to find the other solutions
Reply 6
Avatar for B_9710
B_9710
18
Original post by Think People
I tend to not use the CAST diagram method.
I draw the sin graph out then use symmetry to find the other points.For this type of question do you have any idea on how to do it without the cast method ? :frown:

Here's a plot that may help you.
image.jpg
Original post by Think People
Inverse sin -1/3 gives -19.47... , where do i go from here


That fits within your range but there are other values which sin theta gives you 0 and - 1/3. It's sort of difficult to explain over tsr. You should watch h exam solutions videos on this subject
Original post by Shiv Loves Maths
That fits within your range but there are other values which sin theta gives you 0 and - 1/3. It's sort of difficult to explain over tsr. You should watch h exam solutions videos on this subject


I get it. The sin grpah i drew wasnt accurate.
-1/3 gives -0.3. I drew a line across and see which points meets with the graph.
Thx for the help
(edited 8 years ago)

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you