Question

Determine an expression equivalent to cos(Π+2A)
a.-cos2A
b.cos2A
c.-sin2A
d.sin2A

Answer 1

@lin \(\cos x\) is negative in the third quadrant
Therefore \(\large \cos (\pi+x)=-\cos x\)

Answer 2

but dont i need to exapnd it with addition identities for these kind of questions?

Answer 3

@lin.ivory

Answer 4

Nope not needed here

Answer 5

you saved me twice today!! Thanks. =)

Answer 6

Welcome @lin.ivory

Answer 7

You can do it with the addition identities if proof is required.

Answer 8

\[\cos (\pi+2A)=\cos \pi cos 2A-\sin \pi \sin 2A\]
\[\cos \pi=-1\]
and
\[\sin \pi=0\]
\[\cos (\pi+2A)=-1 \times \cos2A-0=-\cos 2A\]

Answer 9

but for trig equation questions, how do i determine whether what to use? theres the graphs, then the four quadrant ones, and then the identities, how do i know which one would give me the direct answer?

Answer 10

Depending on points. If it's 1 point then just give the answer
If it's more than that use the identities and get to the final result

Answer 11

sorry for all the questions, but what is "1 point" though? you mean like cosx is 1 point and cos2x is two points?

Answer 12

I think he means 1 mark is available for writing a correct answer.

Answer 13

HAHA!! gotcha! thx @IceCreamVan so it just depend on whether i need to show my works or not?! THANKS! =)

Answer 14

Yup! Always show your working when necessary though, one can never tell!

Answer 15

=) of course!