>>729 エラー処理してないけど、こんな感じ？
#include <stdio.h>
#include <math.h>
int main(void)
{
double a, b, c, S, rad;
char buf[80];

printf("a = "); fgets(buf, sizeof(buf), stdin); sscanf(buf, "%lf", &a);
printf("b = "); fgets(buf, sizeof(buf), stdin); sscanf(buf, "%lf", &b);
printf("rad = "); fgets(buf, sizeof(buf), stdin); sscanf(buf, "%lf", &rad);
c = sqrt(pow(a, 2.0) + pow(b, 2.0) + 2 * a * b * cos(rad));
S = (a * b * sin(rad)) / 2.0;
printf("a = %g, b = %g, c = %g, and S = %g\n", a, b, c, S);
return 0;
}

実行例
% ./a.exe
a = 4
b = 3
rad = 1.57
a = 4, b = 3, c = 5.00191, and S = 6

% ./a.exe
a = 2
b = 1.732051
rad = 1.047
a = 2, b = 1.73205, c = 3.23501, and S = 1.49983