CODEWITHSUNDEEP
javamath
Jessica M.
Jessica M.

Reputation: 1459

Using Math class to convert cosine into degrees

I don't know if I have a fundamental misunderstanding of sine, cosine and tangent or I have a misunderstanding of some of the Math class methods or I'm missing something else but I'm little confused.

I have a right triangle ABC with B = 90 degrees. AB is vertical, BC is flat and AC is the hypotenuse. The length value for AB = 125 and AC = 150. I want to find the value of angle A.

enter image description here

I first try to find the cosine and then I want to convert the cosine to degrees. 

double AB = 125;
double AC = 150;


double angleA = Math.cos(AB / AC);
double angleADegrees = Math.toDegrees(angleA);
println(angleADegrees);

The program as it's written returns 57.295. If i try it on my calculator I get a different answer. If I calculate on my calculator to find the cosine = 125/150 i get .8333. Cos^-1(.83333) = 33.5573. The strange thing 57 + 33 + 90 = 180. My program above somehow fnids the value for angle C instead of angle A. Am i calculating for cosine wrong or am I misusing the methods from Math class? I don't know why my program does not return 33 which is what I believe to be the value of angle A.

Upvotes: 3

Views: 18100

Answers (2)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476503

You can convert radians into degrees by multiplying it with 180/Math.PI, Furthermore the inverse cosine (or cos^-1 like some textbooks denote this) is the Math.acos method (a standing for arc).

thus

double AB = 125;
double AC = 150;

double angleA = Math.acos(AB / AC);
println(angleA*180.0d/Math.PI);

will solve the problem.

In case you want to find angle C, you can calculate this by coding:

double angleCDeg = 90.0d-angleA*180.0d/Math.PI;//angle in degrees.
double angleC = angleC*Math.PI/180.0d;//angle in radians.

Upvotes: 5

Preet Sangha
Preet Sangha

Reputation: 65466

mathematically: 

cos A = AB/AC

so cos A = 125 / 150

==> 

A = cos ^-1 ( 125 /150)

So from (http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Math.html)

double angleRad = Math.acos( AB / AC)

and 

double angleDegrees  = angleRad * (180 / Math.PI)

Upvotes: 1

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