Question

can i ask a question in trigo? 1. cos(90degrees-y)=sinx 2. sinsquared A/2=secA-1/2secA 3. cos2A=1-8sinsquared(A/2)cossquared(A/2) 4.sinA=2sin(A/2)cos(A/2) THANKS! :)

Answer 1

what do you want done?

Answer 2

are these identities and you need to prove?

Answer 3

yes. thank you

Answer 4

for the first one I think you mean 90-x correct?

Answer 5

no. it's y. my prof gave that question. im so confused

Answer 6

now, I'm confused...
because this I know cos(90-x)=sinx

Answer 7

do this anyway?

Answer 8

that's why i'm confused. can you answer the 90-x?

Answer 9

i'll just ask my prof about it later. but i think you're correct

Answer 10

here's the first according to what I think the identity is:
cos(90 - x) = cos90*cosx + sin90*sinx by the sum/difference rule for cosine
=0*cosx + 1*sinx because cos90=0 and sin90 = 1
= sinx
QED

Answer 11

thanks

Answer 12

continue? or are we done?

Answer 13

i think its done. next question :)

Answer 14

let's do the easy ones first... #4 next...

Answer 15

ok. :)

Answer 16

that's just the double angle formula for sine...
let M=A/2
so by the double angle formula for sine,
sin(2M) = 2*sinM*cosM

now replace M with A/2 so you have
sin(2*A/2) = 2*sin(A/2)cos(A/2)
sin(A) = 2sin(A/2)cos(A/2)

Answer 17

the last is the final answer?

Answer 18

if you mean the end of the proof then, yes.

Answer 19

thnks my friend. next one? hehehe.

Answer 20

Let's try #3.
btw, did your prof have advice of how to prove identities?

Answer 21

Like, to prove identities, what do you usually start with, the complicated side or the easier side?

Answer 22

Trix, might want to remove some comments there.

Answer 23

you can post here, ok? i'll be back tonight. Thank You. GodBless