Imaginary Numbers - Further Maths Watch

amydowse
Badges: 1
Rep:
?
#1
Report Thread starter 4 years ago
#1
The question is....

Write in the form 'a+ib' where a and b are any real number:

4( cos (-5pi/6) + i sin (-5pi/6))



The answer in the book is:

-2 rt3 - 2i


Any help would be brilliant and I've spent about half an hour trying to figure this one out and I've just ended up getting myself more confused than when I started!!
0
reply
TeeEm
Badges: 19
Rep:
?
#2
Report 4 years ago
#2
(Original post by amydowse)
The question is....

Write in the form 'a+ib' where a and b are any real number:

4( cos (-5pi/6) + i sin (-5pi/6))



The answer in the book is:

-2 rt3 - 2i


Any help would be brilliant and I've spent about half an hour trying to figure this one out and I've just ended up getting myself more confused than when I started!!
type the cosine and the sine in the calculator (in radians)
then multiply by 4
0
reply
Ktulu666
Badges: 4
Rep:
?
#3
Report 4 years ago
#3
(Original post by amydowse)
The question is....

Write in the form 'a+ib' where a and b are any real number:

4( cos (-5pi/6) + i sin (-5pi/6))



The answer in the book is:

-2 rt3 - 2i


Any help would be brilliant and I've spent about half an hour trying to figure this one out and I've just ended up getting myself more confused than when I started!!
Look at the form it's currently in: after you bring the four into the bracket, you get 4cos(-5pi/6) + i(4sin(-5pi/6)). Can you see that's exactly of the form a+bi? cos and sin are always going to be real when you put real numbers into them, so all you need to do is evaluate what the cosine and sine of -5pi/6 is.

Tip: learn the values of cosine and sine at the following values:
0, pi/6, pi/4, pi/3, pi/2, pi. From these, and making use of a sketch graph you can work out any of the other common values you'll need to evaluate. It's very useful to be able to immediately know what these things are without having to reach for a calculator every time.
0
reply
Ilovemaths96
Badges: 13
Rep:
?
#4
Report 4 years ago
#4
(Original post by amydowse)
The question is....

Write in the form 'a+ib' where a and b are any real number:

4( cos (-5pi/6) + i sin (-5pi/6))



The answer in the book is:

-2 rt3 - 2i


Any help would be brilliant and I've spent about half an hour trying to figure this one out and I've just ended up getting myself more confused than when I started!!
have you tried multiplying out the brackets, to get it in the form x+iy (rectangular form)?
0
reply
amydowse
Badges: 1
Rep:
?
#5
Report Thread starter 4 years ago
#5
Thank you so much - I'd been doing a lot longer way!
I didn't realise I could just expand it (rather stupidly)
Thank you
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Cranfield University
    Cranfield Forensic MSc Programme Open Day Postgraduate
    Thu, 25 Apr '19
  • University of the Arts London
    Open day: MA Footwear and MA Fashion Artefact Postgraduate
    Thu, 25 Apr '19
  • Cardiff Metropolitan University
    Undergraduate Open Day - Llandaff Campus Undergraduate
    Sat, 27 Apr '19

Have you registered to vote?

Yes! (315)
37.77%
No - but I will (64)
7.67%
No - I don't want to (62)
7.43%
No - I can't vote (<18, not in UK, etc) (393)
47.12%

Watched Threads

View All