（インテルCPUの）三角関数は不要 http://blog.unfindable.net/archives/8991

MyCos.java

/*
<dependencies>
    <dependency>
        <groupId>org.apfloat</groupId>
        <artifactId>apfloat</artifactId>
        <version>1.8.2</version>
    </dependency>
</dependencies>
*/
import org.apfloat.*;

public class MyCos {

  public static void main(String[] args) {
    int c1BetterCase = 0;
    int c2BetterCase = 0;
    final int n = 10000;
    final long precision = 30;
    for (int i = 0; i < n; ++i) {
      double x = Math.random();
      double c1 = Math.cos(x);
      double c2 = StrictMath.cos(x);
      Apfloat c = ApfloatMath.cos(new Apfloat(x, precision));
      if (c1 != c2) {
        Apfloat delta1 = ApfloatMath.abs(c.subtract(new Apfloat(c1, precision)));
        Apfloat delta2 = ApfloatMath.abs(c.subtract(new Apfloat(c2, precision)));
        if (delta1.compareTo(delta2) < 0) {
          ++c1BetterCase;
        } else {
          ++c2BetterCase;
        }
        System.out.println("x = " + x);
        System.out.println("c1 = Math.cos(x)        = " + c1);
        System.out.println("c2 = StrictMath.cos(x)  = " + c2);
        System.out.println("c  = ApfloatMath.cos(x) = " + c.toString(true));
        System.out.println("abs(c - c1) = " + delta1);
        System.out.println("abs(c - c2) = " + delta2);
        System.out.println();
      }
    }
    System.out.println("c1 is better: " + c1BetterCase);
    System.out.println("c2 is better: " + c2BetterCase);
    System.out.println("total: " + n);
  }
}