Trigonometry

posted by .

Am I doing something wrong? I have to use the Law of Consines with the following numbers: a=30, b=15, c=17. I tried to find the angle a, since a is the longest side. Cos A=(15^2+17^2-30^2)/(2x17x15)=-.7568 The inverse being 139 degrees. But when I multiply 15, the measure of b by sin 139/30, I get .32 or 18.66. This is 157. 180-157=23 But when I multiply 17 by sine139/30 I get 21. Does this mean there are two triangles, or that I did it wrong?

  • Trigonometry -

    a^2=b^2+c^2-2bcCosA

    CosA= 1/2 * (b^2+c^2-a^2)/bc
    A= 139 deg, you are right on that.

    Lets find angle B, by the law of sines

    sinB/b=SinA/a or sinB= .3269 or B=19 deg
    Lets find angle C by the law of sines

    SinC/c=SinA/a or SinC=.3718 C=21.8 deg

    check: do the sum of angles = 180 deg?
    139+21.8+19=180 rounded.

    All is well.

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. geometry

    I am studying for my GRE exam into grad school and it's been a very long time since I've done geometry. I have a problem to which I need to solve for the area of a triangle but I do not have the base. I do have all angles but I cannot …
  2. Trigonometry

    There is an arbitrary triangle with angles A, B, and C and sides of lengths a, b, and c. Angle A is opposite side a. How do I get the formulas: b * cos C + c * cos B = a c * cos A + a * cos C = b a * cos B + b * cos A = c Are these …
  3. Trigonometry

    Prove the identity sin(x+y+z)+sin(x+y-z)+sin(x-y+z)+ sin(x-y-z) = 4 sin(x)cos(y)cos(z) This identity is so long and after i tried to expand the left side and it just looked something crap Thanks for you help :)
  4. trigonometry

    using the law of sines, solve the rest of the triangle: side C=14 side B=4 angle c=145 degrees i got angle b= 9.4 angle a=25.6 side A=10.4 but i didn't know if it was correct
  5. trigonometry (repost) Mathmate

    I want to start by saying thank you . You have no idea how much u have helped me understand logarithms, even better then the books i have (it poorly explains the subject of trigonometry let alone logarithms and antilog). Your last …
  6. Trigonometry (repost Reiny)

    at 1:35am I posted ; Write equivalent equations in the form of inverse functions for a.)x=y+cos theta b.)cosy=x^2 my answers were a.) x= y+ cos theta cos theta = x-y theta = cos^-1(x-y) b.) cosy=x^2 cos(y) = x^2 y = Cos^-1(x^2) your …
  7. trig

    use an inverse trig function to write è as a function of x (There is a right triangle. The hypotenuse is 2. è is the angle between the base and the hypotenuse. The base/adjacent side of è is (x + 1) The side opposite è does not …
  8. Geometry

    Use the special triangles to find the exact values of the following: 1) csc45degrees 2)sec30degrees 3)longest side of a 30-60-90 triangle if the side opposite the 60degree angle is 5radical3
  9. physics

    "Rocket Man" has a propulsion unit strapped to his back. He starts from rest on the ground, fires the unit, and accelerates straight upward. At a height of 16 m, his speed is 5.5 m/s. His mass, including the propulsion unit, has the …
  10. Math

    An angle θ satisfies the relation csc θ cos θ = -1. A) Use the definition of the reciprocal trigonometric ratios to express the left side in terms of sin θ and cos θ. B) What is the relation between sin θ and cos θ for this …

More Similar Questions