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Homework Help: Trigonometry- addition and factor forumla

  1. Nov 23, 2012 #1
    1. The problem statement, all variables and given/known data
    If tan(A+B) = 3 and tan(A-B) = 2, find tan2A and tan2B

    2. Relevant equations
    tan (A - B) = (tan A - tan B)/(1 + (tan A)(tan B) and similar sort of one for tan(A+B)

    3. The attempt at a solution
    i did some calculation and got tanA= (1-2tanB)/5tanB

    after which there seems some thing missing for the proceeding calculations....
    wonder what to do next?
  2. jcsd
  3. Nov 23, 2012 #2


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    Staff Emeritus
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    Homework Helper

    For better service, try posting your math questions in either the Precalculus math or the Calculus sections of the HW forums.
  4. Nov 23, 2012 #3


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    Staff: Mentor

    Topic moved. As SteamKing wrote - this is definitely not "OtherSciences", but Math itself.
  5. Nov 23, 2012 #4
    The "other identity" of "the sort" you are talking about is given like this:


    Now, a good start to this question would be to write everything you already have down: (name tan(a)=x and tan(b)=y) [itex]\displaystyle 3=\frac{x+y}{1-xy}[/itex] and [itex]\displaystyle 2=\frac{x-y}{1+xy}[/itex]. Then, these are two equations with two unknowns (a and b.) Try to manipulate the expressions and eliminate a term that you would not want in an equation with two unknowns. The rest follows relatively easily.

    If you found some value for tan A whose inverse tangent is not very pleasant, you are doing something wrong; the value A comes out very nicely.

    Tip: Your expression for tan A is not the simplest one possible. Try to find linear expressions.
    Tip-2: There isn't only one solution.
    Last edited: Nov 23, 2012
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