Question

cosine rule : cos(a)=-1/sqrt(2).. how do i evaluate this ? can a = 3/4pi and 5/4pi ?

Answer 1

have you tried taking the arc cosine of both sides?

Answer 2

yes...

Answer 3

or are you supposed to use fancy complicated trigonometric formulas?

Answer 4

3/4pi and 5/4pi both answers?

Answer 5

yes...do u know this?

Answer 6

YES

Answer 7

how can an angle in triangle = 5/4pi ?

Answer 8

see in 2nd and 3rd quadrant cos will be negative....and you have not been asked for a triangle

Answer 9

no i was using the cosine rule to find angle ..

Answer 10

sorry my bad.. i was finding angle in triangle using cosine rule and got what istated in queston

Answer 11

and u r aware with the identities
cos(pi-theta) = -cos(theta)
cos(pi+theta) = - cos(theta)

Answer 12

YES i solved it already .. 3pi/4 and 5pi/4

Answer 13

good...:)
then u know...how to solve this type of question?

Answer 14

im not asking how to solve sine equations.. just when using cosine rule to find angle in a triangle as i did.. are both 3pi/4 and 5pi/4 possible angles ? for say alpha

Answer 15

okay so it was originally \[(\sqrt{10})^{2} = (\sqrt{2})^{2} + 2^2 -2(2)(\sqrt{2})\cos(x) \]
which simplified to \[\cos(x)=-\frac{1}{\sqrt{2}} \]

Answer 16

oh ok...then rule out that angle...it can't be....sorry

Answer 17

so x=3pi/4,5pi/4 but angles in triangle<pi .: 5pi/4 not soln?

Answer 18

yes...u r right

Answer 19

if u r being asked in a triangle then x=3 pi/4

Answer 20

hm ok..

Answer 21

and uh.. how to solve tan equations? like tan(x)=-1 not quite sure

Answer 22

use the identity
tan(pi - theta) = -tan(theta)

Answer 23

do i get x=3pi/4 and 7pi/4 and keep adding 2pi to each one ?

Answer 24

or tan( 2 pi -theta) = -tan(theta)

Answer 25

yes...actually when
tan theta= x
then x= n(pi) + theta, where n = ....-1,-2,-1,0,1,2,3....

Answer 26

yeah im just wondeirng do u keep adding pi or get the 2 angles and add 2 pi.. or same thing

Answer 27

aksh??

Answer 28

actually tan has a period pi....