If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)

Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie

Trigonometric Identity Proofs Question

[Deleted User] · 2009-05-30 14:19:50

If the question for the proof of, just say, sin2A = 2sinAcosA came up, are you allowed to just sub A in for B into the sin(A+B) = sinAcosB+cosAsinB formula? Like I know that's how it's derived, but, are you allowed to assume that the sin(A+B) formula is true? Or do you've to derive that before you use it (as it's proof is…

Options

30-05-2009 3:19pm

#1

[Deleted User]
Posts: 4,630 ✭✭✭

Join Date: -

Posts: 4286

If the question for the proof of, just say, sin2A = 2sinAcosA came up, are you allowed to just sub A in for B into the sin(A+B) = sinAcosB+cosAsinB formula? Like I know that's how it's derived, but, are you allowed to assume that the sin(A+B) formula is true? Or do you've to derive that before you use it (as it's proof is on the syllabus, too).

0

Comments

Options
#2 30-05-2009 3:34pm
Fringe

Registered Users Posts: 1,082 ✭✭✭

Join Date: November 2006

Posts: 1043

Look on the log tables. You are allowed to use the formulae before that one to prove it. sin2A is after sin(A + so yes, you can use that to prove it.

0
Options
#3 30-05-2009 4:00pm
[Deleted User]
Posts: 4,630 ✭✭✭

Join Date: -

Posts: 4286

Oh right right, I didn't know that about the order. Thanks Fringe.

0