Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

## #1 2010-05-11 15:59:50

mickeen
Member

Offline

### Trigonometry

(a) Express Sin2A + Sin2B as a product in Sine and Cosine.

(b) If A + B + C = 180 degrees,
show that Sin (A + B) = Sin C

(c) Hence show that Sin 2A + Sin 2B _ Sin 2C = 4CosACosBSinC.

Note: Cos(A + B) = - CosC

Can anyone plese help me on the last part of this question which has bothered me for some time now.

## #2 2010-05-11 16:04:01

bobbym

Offline

### Re: Trigonometry

Hi mickeen;

a) First thing that comes to mind is:

b) If A + B + C = 180 then A + B = 180 - C and

sin( 180 - c ) = sin(180° )cos(c) - cos(180° )sin(c)

= sin(c)

For c)

(c) Hence show that Sin 2A + Sin 2B _ Sin 2C = 4CosACosBSinC.

What does the underscore mean?

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #3 2010-05-11 16:34:45

ZHero
Real Member

Offline

### Re: Trigonometry

(a) Express Sin2A + Sin2B as a product in Sine and Cosine.

Last edited by ZHero (2010-05-11 18:08:26)

If two or more thoughts intersect with each other, then there has to be a point.

## #4 2010-05-11 17:28:04

bobbym

Offline

### Re: Trigonometry

Hi mickeen;

For C)

Sin 2A + Sin 2B _ Sin 2C = 4CosACosBSinC.

I going to assume you meant:

remember sin(C) = sin(A+B)

See the result I gave you in post #6, Use it right here.

Done!

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #5 2010-05-11 18:09:00

ZHero
Real Member

Offline

### Re: Trigonometry

(b) If A + B + C = 180 degrees,
show that Sin (A + B) = Sin C

If two or more thoughts intersect with each other, then there has to be a point.

## #6 2010-05-11 18:26:44

bobbym

Offline

### Re: Trigonometry

Hi mickeen;

For a) this came to me while doing c)

http://www.sosmath.com/trig/Trig5/trig5/trig5.html

The sum to product formulas:

Just say u = 2A and v = 2B

And you get:

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #7 2010-05-16 04:52:02

mickeen
Member

Offline

### Re: Trigonometry

Thanks a million for this. I have printed it off and will try to digest it over a bowl of soup and a glass of wine later tonight!

mickeen

## #8 2010-05-16 04:53:49

bobbym

Offline

### Re: Trigonometry

Hi mickeen;

Enjoyed working on c), thanks for posting it. Save me some soup.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #9 2010-05-18 06:46:10

mickeen
Member

Offline

### Re: Trigonometry

Bobby M,

thanks for your help again this time! Dont know what I would do without you! I have it all written out now again (your explanation) and understand it perfectly. Are you any good on Stats? I am OK a while but might have a few questions in June. The soup and the wine was nice. But all gone by the end of the 80 mile cycle yesterday morning! I suppose one cant really share cyber soup! However! Thanks again!

Mickeen

## #10 2010-05-18 06:51:31

bobbym

Offline

### Re: Trigonometry

Hi mickeen;

You mean I got some of that right!!!!! I knew those Tarot cards worked.

Thanks man. I am glad you got it. I can do some stats and I like it. I am unusually good in stats, getting half of the questions right, provided it is a 2 choice per question test. I know what you would do without me, better! Bring it in and if I am around I will help. Don't worry about the soup.

Just let me say, thanks for saying those kind things, lately I have been feeling pretty unappreciated by some people.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.