Question

No idea how to work this question.
Not sure which identities and formulas to use.
If a and b are two angles in Quadrant II such that tan a=-1/2 and tan b=-2/3, find cos(a+b).

Answer 1

@SolomonZelman @thomaster

Answer 2

any help would be appreciated!

Answer 3

^.^

Answer 4

im wondering if my question is too difficult or too easy! or just not a poplular subject right now :)

Answer 5

too hard or too easy?
lol why?
what do you think of it yourself? ^.^
`

Answer 6

if im asking for help... i think it is pretty hard! :l

Answer 7

:)

Answer 8

nothing's hard if you'll only make it easy :D
think of it as a game ^.^

Answer 9

yeah... one i dont know the rules too ;)

Answer 10

rules are for grown-ups >:)
what we need is...well something, but it isn't rules...it's
sheer awesomeness :D

Answer 11

which I happen to have... (ha ha ha ha >:) )

anyway, the most obvious thing to do is expand
cos(a+b)

like... cos(a)cos(b) - sin(a)sin(b)

Answer 12

and an absence of trig... ok let me look at what that does.

Answer 13

okay so sum and difference formula.

Answer 14

um... what next?

Answer 15

Okay, I figured it out. Thanks for sending me in the right direction!