a1=4;
a2=4.5;
a3=5;
eps=1e-7;
y1=sin（a1）;
y2=sin（a2）;
y3=sin（a3）;
h=1;
c1=（y3-y1）/（a3-a1）;
c2=（（y2-y1）/（a2-a1）-c1）/（a2-a3）;
ap=0.5*（a1+a3-c1/c2）;
yp=sin（ap）;
while?abs（（y2-yp）/y2）>=eps
????if?（ap-a2）*h>0
????????if?y2>=yp
????????????a1=a2;y1=y2;a2=ap;y2=yp;
????????else
????????????a3=ap;y3=yp;
????????end
????e