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# Trigo Help pls watch

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1. 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?
2. (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
3. (Original post by wonkey)
but sin 2x = 2 sinx cosx, not 2 sin2x cos2x
sin 4x = 2 sin 2x cos 2x
4. (Original post by theone)
sin 4x = 2 sin 2x cos 2x
yeah sorry, i jsut realised. thanx
5. (Original post by wonkey)
yeah sorry, i jsut realised. thanx
No prob
6. (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
7. (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?
8. Is this P3?
9. nopes its p2
i never said it was p3
10. (Original post by MalaysianDude)
nopes its p2
i never said it was p3
Nor did I, I merely asked.
11. erm...so can anyone answer this question?
i need to really solve it...someone help me pls
12. (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.
13. (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
14. (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

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