how to do this question- help (1 Viewer)

philbw

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hey guys its a simple q but for some reason im finding trouble
1603927371692.png
answer is B
can someone give an explanation ty
 

erucibon

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Draw a triangle, use pythag, expand sin2x into 2sinxcosx, since x is obtuse its in the second quadrant so sin is positive and cos is negative, use the triangle to evaluate cos considering the sign.
 

Trebla

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Or you could just solve for x to find the obtuse angle directly and then calculate sin 2x
 

cossine

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We know from the special ratios sin(60) = sqrt(3)/2

Sine is defined on the unit circle as sin theta = y. The obtuse angle is 120 as the related angle of 120 is 60 degrees. Note that sine is positive in the second quadrant(ASTC).

sin(120*2) = sin(240).

The related angle of 240 degrees is 60 degrees but sine is negative in the third quadrant as all the y values are negative in the third quadrant(see definition or remember ASTC).

Therefore sin(240) = - sqrt(3)/2
 

Alistruggles

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We know from the special ratios sin(60) = sqrt(3)/2

Sine is defined on the unit circle as sin theta = y. The obtuse angle is 120 as the related angle of 120 is 60 degrees. Note that sine is positive in the second quadrant(ASTC).

sin(120*2) = sin(240).

The related angle of 240 degrees is 60 degrees but sine is negative in the third quadrant as all the y values are negative in the third quadrant(see definition or remember ASTC).

Therefore sin(240) = - sqrt(3)/2
how did you go from sqrt3/4 to this?

edit: i'm dumb this is a different question lol i'm looking at Q2 of sydney girls trials which is the exact same but Question is sqrt 3/4
 

Alistruggles

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hey guys its a simple q but for some reason im finding trouble
View attachment 29304
answer is B
can someone give an explanation ty
the answer is A according to my copy ?

edit: i'm dumb this is a different question lol i'm looking at Q2 of sydney girls trials which is the exact same but Question is sqrt 3/4
 

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