Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta

Trigo Help pls watch

Announcements
    • Thread Starter
    Offline

    0
    ReputationRep:
    yo all..
    just wondering if you can help me with this
    Sin4X + Sin2X = 0
    i used the sum and product formula to get
    2Sin3XCosX = 0

    then im stuck
    can someone help me out?
    Offline

    0
    ReputationRep:
    (Original post by MalaysianDude)
    yo all..
    just wondering if you can help me with this
    Sin4X + Sin2X = 0
    i used the sum and product formula to get
    2Sin3XCosX = 0

    then im stuck
    can someone help me out?
    The easiest approach is:

    sin 4x = - sin 2x

    2 sin 2x cos 2x = - sin 2x

    sin(2x) (1+2cos(2x)) = 0.

    So sin(2x) = 0 => x = 0, 90, 180, etc.

    Or cos(2x) = -1/2. => x = 60, 120, etc
    Offline

    0
    ReputationRep:
    (Original post by wonkey)
    but sin 2x = 2 sinx cosx, not 2 sin2x cos2x
    sin 4x = 2 sin 2x cos 2x
    Offline

    0
    ReputationRep:
    (Original post by theone)
    sin 4x = 2 sin 2x cos 2x
    yeah sorry, i jsut realised. thanx
    Offline

    0
    ReputationRep:
    (Original post by wonkey)
    yeah sorry, i jsut realised. thanx
    No prob
    Offline

    1
    ReputationRep:
    (Original post by theone)
    The easiest approach is:

    sin 4x = - sin 2x

    2 sin 2x cos 2x = - sin 2x

    sin(2x) (1+2cos(2x)) = 0.

    So sin(2x) = 0 => x = 0, 90, 180, etc.

    Or cos(2x) = -1/2. => x = 60, 120, etc
    How did you get to:

    sin(2x) (1+2cos(2x)) = 0
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by theone)
    The easiest approach is:

    sin 4x = - sin 2x

    2 sin 2x cos 2x = - sin 2x

    sin(2x) (1+2cos(2x)) = 0.

    So sin(2x) = 0 => x = 0, 90, 180, etc.

    Or cos(2x) = -1/2. => x = 60, 120, etc
    is this the only method of doing it?
    i mean..i know this method but the question wanted me to use the sum and product formula
    which is

    2Sin((X+Y)/2)Cos((X-Y)/2)

    from SinX and SinY
    how do you use this formula and do it?
    Offline

    1
    ReputationRep:
    Is this P3?
    • Thread Starter
    Offline

    0
    ReputationRep:
    nopes its p2
    i never said it was p3
    Offline

    1
    ReputationRep:
    (Original post by MalaysianDude)
    nopes its p2
    i never said it was p3
    Nor did I, I merely asked.
    • Thread Starter
    Offline

    0
    ReputationRep:
    erm...so can anyone answer this question?
    i need to really solve it...someone help me pls
    Offline

    1
    ReputationRep:
    (Original post by MalaysianDude)
    yo all..
    just wondering if you can help me with this
    Sin4X + Sin2X = 0
    i used the sum and product formula to get
    2Sin3XCosX = 0

    then im stuck
    can someone help me out?
    2sin3xcosx = 0
    sin3xcosx = 0

    Then sin3x = 0 or cosx = 0, and solve.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by XTinaA)
    2sin3xcosx = 0
    sin3xcosx = 0

    Then sin3x = 0 or cosx = 0, and solve.
    erm..
    you sure thats the way?
    argh..
    iw as so careless :P
    heehhe

    thx anyways
    Offline

    0
    ReputationRep:
    (Original post by Bhaal85)
    How did you get to:

    sin(2x) (1+2cos(2x)) = 0
    2sin 2x cos 2x = - sin 2x.

    sin 2x + 2 sin 2x cos 2x = 0

    sin 2x (1+2cos 2x) = 0
 
 
 
Poll
Do you like carrot cake?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.