how to do this question- help (1 Viewer)

[philbw]

hey guys its a simple q but for some reason im finding trouble

answer is B
can someone give an explanation ty

 

[erucibon]

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]

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

 

[cossine]

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]

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]

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