Question

If cosA= -15/17 and A is in quadrant III, find cos 1/2A

Answer 1

ya you have the value of cos A
try to use the formula of cos 2A to get to the answer!!

Answer 2

@sarihmanlapaz did you try that?

Answer 3

what will i use in the 3 formula of cos2a?

Answer 4

just look at the equations and look at what we are trying to get the answer in terms of

Answer 5

we want cos A/2 isnt it?

Answer 6

and we have cos A , now you tell me which equation to use!!

Answer 7

i think cos^2 - 1

Answer 8

\[2\cos^2 x-1\]

Answer 9

so how will i substitute it to that equation?

Answer 10

cosA=2cos^2(A/2)-1
find cos(A/2) from here.

Answer 11

@sarihmanlapaz gt it?

Answer 12

got it! thanks :)

Answer 13

but its none of the choices. >.< how can i solve it?

Answer 14

what did u get ?

Answer 15

0.2435

Answer 16

i m getting -0.2435, is that the option ?

Answer 17

oh no. haha my answer 0.2435 is in the options. i did'nt notice it.

Answer 18

but A/2 will be in 2nd quadrant where cos is negative.
so we put minus
-0.2435

Answer 19

its written in the question that the A is in quadrant 3

Answer 20

yes, A in 3rd,A/2 in 2nd.
is -0.2435 in options?

Answer 21

the options are. A).0.24054 B).0.24125 C) 0.24254 D) 0.24354

Answer 22

pls can you help me?

Answer 23

since all options are positive, i guess A/2 goes in 1st quadrant...then 0.2435 is correct.

Answer 24

no. its wrong my friend. i just check the half angle formula. haha there is a formula for cosA/2 = squareroot of 1/2(1+A)

Answer 25

u mean ....(1+cos A) ?

Answer 26

yes. yes :) i got the right answer 0.2425

Answer 27

how?
the formula we gave and what u wrote are exactly the same...

Answer 28

http://www.themathpage.com/atrig/trigonometric-identities.htm

Answer 29

there is the formula for half angle :)

Answer 30

so using that u got different answer?
we gave u that formula only.....
cosA=2cos^2(A/2)-1
so
cos(A/2)=sqrt{0.5*(1+cos A)}

Answer 31

yeah. thanks i got the right answer.