Posted by **Riley** on Sunday, November 14, 2010 at 10:00pm.

If sin(theta)=15/17 and cos(beta)=(-5/13 (both theta and beta are in quadrant II) find tan(theta+beta)

- Pre-Calculus -
**Reiny**, Sunday, November 14, 2010 at 10:30pm
You should recognize the 8,15,17 and 5,12,13 right-angled triangles

so in II if

sinØ= 15/17, then cosØ = -8/17 and tanØ = - 15/8

cosß = -5/13 , then sinß = 12/13 and tanß = -12/5

tan(Ø+ß) = (tanØ + tanß)/(1 - tanØtanß)

= ....

you finish the arithmetic.

- Pre-Calculus -
**Riley**, Sunday, November 14, 2010 at 10:41pm
Ok i understand until you say that tan(theta) +tan(beta) needs to be divided by 1-tan(theta)tan(beta) where does that come from?

- Pre-Calculus -
**Reiny**, Sunday, November 14, 2010 at 10:54pm
I assumed you knew the formulas, since you have know them to do these kind of problems

look under "other identities"

http://library.thinkquest.org/20991/alg2/trigi.html

- Pre-Calculus -
**Daniel**, Sunday, November 14, 2010 at 11:00pm
I do but why did u use that one. Im trying to understand because i have a test tomorrow and i have to do well on it.

- Pre-Calculus -
**Reiny**, Sunday, November 14, 2010 at 11:18pm
well, it asked for tan(Ø+ß) didn't it?

The letters themselves of course don't matter

e.g.

tan(A+B) =(tanA + tanB)/(1- tanAtanB)

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