Thread: Double-Angle Formulas

Hello,

Can someone please help me with these problems?

Find Sin2x, Cos 2x, and Tan2x for:

1. tan x= -4/3 quadrant II
2. csc x = 4, tan x<0

how do I start the problem?

for 1, is Sin -4/1 and Cos 3/1
and for 2 is sin 1/4?

Thanks

Hello l flipboi l

Originally Posted by l flipboi l

Hello,

Can someone please help me with these problems?

Find Sin2x, Cos 2x, and Tan2x for:

1. tan x= -4/3 quadrant II
2. csc x = 4, tan x<0

how do I start the problem?

for 1, is Sin -4/1 and Cos 3/1
and for 2 is sin 1/4?

Thanks

Are you familiar with this diagram that tells you when the various trig functions are positive?

(Remember ACTS starting in QI and going clockwise.)

1. In QII, only sine is positive. So if and is in QII, and (using Pythagoras). You should be able to find the sine, cosine and tangent of using the standard double-angle formulae.

2. You are right: . Since sine is positive, we are either in QI or QII. So the fact that means that must be in QII. Using Pythagoras, then:

and

Again, the standard double-angle formula will give you the results you need.

Grandad

Originally Posted by Grandad

Hello l flipboi l
Are you familiar with this diagram that tells you when the various trig functions are positive?

(Remember ACTS starting in QI and going clockwise.)

1. In QII, only sine is positive. So if and is in QII, and (using Pythagoras). You should be able to find the sine, cosine and tangent of using the standard double-angle formulae.

2. You are right: . Since sine is positive, we are either in QI or QII. So the fact that means that must be in QII. Using Pythagoras, then:

and

Again, the standard double-angle formula will give you the results you need.

Grandad

Thanks Grandad!