OCR (not MEI) C2 - Wednesday 20th May 2015

What happens when you are doing trigonometry and you get a negative angle ?

Reply 62

Original post by Kadak

What happens when you are doing trigonometry and you get a negative angle ?

Do you have a question as an example?

Reply 63

Sahil_

Original post by Kadak

Where did you get the 272 or the log after it ?

Sorry I forgot to press enter :P,
y²=27
log₃y² = log₃27
2log₃y = 3
log₃y = 1.5

Reply 64

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20QP%20-%20C2%20OCR.pdf
Question 5(ii) When I got my sin angles ,I got sin^-1(-2/5) which got me a negative angle.

Reply 65

sandpitturtle

6

Original post by Kadak

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20QP%20-%20C2%20OCR.pdf
Question 5(ii) When I got my sin angles ,I got sin^-1(-2/5) which got me a negative angle.

use cast diagram to find the positive angle then keep going like a normal question like this

Reply 66

Quick question ?Do we have to show the CAST diagram ?Can we draw two ,one for each angle Im finding ?

Reply 67

Original post by Kadak

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20QP%20-%20C2%20OCR.pdf
Question 5(ii) When I got my sin angles ,I got sin^-1(-2/5) which got me a negative angle.

That is no problem. Can you please give me a min to write it up :smile:

Reply 68

giantbecky

Can anyone help me with this log question please?

Find the roots of the equation 2log(base4)x - log(base4)(-7x+3)=-1/2?

I have got to 2x^2 + 7x - 3 = 0 but you have to use the quadratic formula to factorise this and I do not get the correct answer...have I made a mistake?

Thanks!

Reply 69

Original post by Kadak

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20QP%20-%20C2%20OCR.pdf
Question 5(ii) When I got my sin angles ,I got sin^-1(-2/5) which got me a negative angle.

Reply 70

Original post by Issy :)

I use the CAST method :frown:

. Im confused ,I thought we woud add 180 to 19.5.For the engative angle,did you substract from 180 ?

Reply 71

Original post by Kadak

I use the CAST method :frown:

. Im confused ,I thought we woud add 180 to 19.5.For the engative angle,did you substract from 180 ?

Do you have the answers? I have been doing maths all day so might have messed it up. My brain is literally fried. :wink:

(edited 8 years ago)

Reply 72

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20MS%20-%20C2%20OCR.pdf

Reply 73

Sahil_

Original post by giantbecky

Can anyone help me with this log question please?

Find the roots of the equation 2log(base4)x - log(base4)(-7x+3)=-1/2?

I have got to 2x^2 + 7x - 3 = 0 but you have to use the quadratic formula to factorise this and I do not get the correct answer...have I made a mistake?

Thanks!

I got that equation as well, can you tell what paper this question is from.

Reply 74

giantbecky

Original post by Sahil_

I got that equation as well, can you tell what paper this question is from.

It's not from a paper, it's from this revision guide/textbook...I can tell you the answers if you want?
I thought maybe the answers were wrong but when I put them back into the original log equation they give -1/2 :frown:

Reply 75

https://68baf9f6baae001b7abe18b6e880c559742888e3.googledrive.com/host/0B1ZiqBksUHNYUlVQUzA0enkzVFU/Specimen%20MS%20-%20C2%20OCR.pdf

No, I was right, I just got fed up writing out all the answers halfway through. I really can't explain the cast method as I have always been taught the graph way. Let me think a mo and see if I can explain it :smile:

Original post by Kadak

Reply 76

Sahil_

Original post by giantbecky

It's not from a paper, it's from this revision guide/textbook...I can tell you the answers if you want?
I thought maybe the answers were wrong but when I put them back into the original log equation they give -1/2 :frown:

If you put the answers from the textbook into the equation does is also give -1/2?

Reply 77

Ok, so the 19.5 value is positive so can only be in the first and second quadrant yes (CAST so only A and S) therefore is 180-19.5.
The negative value, for sin to be negative it must be in the 3rd or 4th quadrant. Therefore, ignore the negative when typing into the calc then do 360-23.6 and 180+23.6
That is using the cast diagram (needed a min to jog my memory of the method)

Original post by Kadak

I use the CAST method :frown:

. Im confused ,I thought we woud add 180 to 19.5.For the engative angle,did you substract from 180 ?

Reply 78

giantbecky

Original post by Sahil_

If you put the answers from the textbook into the equation does is also give -1/2?

the answers are 3 and 1/2 and actually I just tried 1/2 and I don't think that works as -7x1/2 +3 = -1/2 and you can't take logs of negative numbers...

So maybe the textbook is wrong? :s-smilie:

Reply 79