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Math (trigonometry)

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How do you get the second triangle when you have an ambiguous case of sine law? For example, if you are given the following information for a triangle:

a = 7.2 mm, b = 9.3 mm, <A = 35°

I can solve for one triangle:

Find <B, <C, side c.

<B:
sinB/9.3 = sin35°/72
sinB = 9.3sin35°/7.2
B = sin^-1 (9.3sin35°/7.2)
B = 48°

<C:
180°-35°-48° = 97°

Side c:
c/sin97° = 7.2/sin35°
c = 7.2/sin97°/sin35°
c = 12.5

How do you find and solve for the second one?

  • Math (trigonometry) -

    your calculator gave you 47.80.. or 48°
    then angle C = 180-35-48 = 97 , you had that correct
    c/sin97 = 7.2/sin35
    c = 7.2sin97/sin35
    = 12.5

    So triangle ABC is such that
    a= 7.2 --- given
    b = 9.3 -- given
    c = 12.5
    Angle A = 35 -- given
    Angle B = 48
    Angle C = 97

    but remember the sine is positive in quadrants I or II
    so angle B could also have been 180-48 or 132 °

    so c/sin48 = 7.2/sin35
    c = 7.2sin48/sin35 = 9.3

    so angle C = 180-132-35 = 13°

    then c/sin13 = 7.2/sin35
    c = 7.2sin13/sin35 = 2.8

    second case
    a=7.2
    b=9.3
    c= 2.8
    A=35°
    B = 132°
    C = 13°

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