平行

#include <stdio.h>
#include <math.h>

main( )
{
float a1,a2,a3;
float b1,b2,b3;
float theta;

a1 = 1;
a2 = 0;
a3 = 0;

b1 = -1;
b2 = 0;
b3 = 0;

theta = (a1*b1+a2*b2+a3*b3)/sqrt(a1*a1+a2*a2+a3*a3)/sqrt(b1*b1+b2*b2+b3*b3);

printf( " theta = %f\n ",theta);
}