Question

Complex numbers HELP!!

Answer 1

@Peter14

Answer 2

Please copy and past your question.

Answer 3

i'm here

Answer 4

Can you please help me?

Answer 5

I hope so

Answer 6

the summation sign in all the answers doesn't really make sense to me right now because it doesn't have any bounds. Can you clarify that for me?

Answer 7

\[\sum_{}^{}\cos2A = \cos2A + \cos2B + \cos2C\]

Answer 8

Similarly
\[\sum_{}^{}\cos(A+B) = \cos(A+B) + \cos(B+C) + \cos(A+C)\]

Answer 9

I see

Answer 10

try to think of a set of values for a, b, and c that satisfy the initial equation and do not satisfy each of the choices
use process of elimination

Answer 11

Actually all choices are correct (this is what stumped me!). So you cannot use elimination.

Answer 12

wait, what?

Answer 13

confusing.

Answer 14

so then what's the problem? proving it?

Answer 15

I think so

Answer 16

take the summation terms and simplify them with identities

Answer 17

Which identities?

Answer 18

trig identities for sin2x, cos2x, sin(a+b) and cos(a+b)

Answer 19

you can look them up in your textbook or just google "trig identities"

Answer 20

I remember them. But how do i use them in this equation?

Answer 21

use them inside the summation to simplify it

Answer 22

if you're trying to prove each one, you're trying to prove that the summation =0

Answer 23

And how can i do that?

Answer 24

use the equation provided at the top and the trig identities

Answer 25

sorry, I have to go to bed now, it's nearly midnight here in China

Answer 26

Goodnight.