IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: zabady on March 28, 2010, 06:14:58 pm

Title: problem in basics of trigonometry
Post by: zabady on March 28, 2010, 06:14:58 pm

i kno that if they give me sinx=1/2 i first get the sin inverse to get the angle then check the sign of (+1/2) and find which quadrant it is then find the other angles example 180-30=150 and 30 in the range of 0 less than or equal x less than 180

but the problem i dnt get is in that question it says cosx=+ or - sq root 3/4 interval 0 less than or equal x less than 360

answers were 30,150,220,330

i dnt understand how to get those answerz

Title: Re: problem in basics of trigonometry
Post by: Saladin on March 28, 2010, 06:24:19 pm

it is basically like this:

(http://www.sitmo.com/gg/latex/latex2png.2.php?z=120&eq=cos(x)%3D\pm(\sqrt{\frac{3}{4}}))

This basically means that, cos(x), can be both positive and negative.

So, its like doing two questions in one. You have to solve for both the positive values and the negative values.

So solve for: (http://www.sitmo.com/gg/latex/latex2png.2.php?z=120&eq=cos(x)%3D%2B\sqrt{\frac{3}{4}})

and solve for:
(http://www.sitmo.com/gg/latex/latex2png.2.php?z=120&eq=cos(x)%3D-\sqrt{\frac{3}{4}})

So, from each you will get two values in the range of 0 to 360 degrees.

And thus you will have a total of 4 values:

(http://www.sitmo.com/gg/latex/latex2png.2.php?z=120&eq=cos(x)%3D%2B\sqrt{\frac{3}{4}})

will give you: 30 and (360-30)=330

and

(http://www.sitmo.com/gg/latex/latex2png.2.php?z=120&eq=cos(x)%3D-\sqrt{\frac{3}{4}})

will give you:

150 and 210.

220 is wrong by the way.

Title: Re: problem in basics of trigonometry
Post by: zabady on March 28, 2010, 06:44:43 pm

thx so much ;D