都用余弦定理。
1、圆心角余弦值为(630^2+630^2-390^2)/2x630^2=713/882
弧长为3.14x630x(arccos713/882)/180=1978.2x(arccos713/882)/180=1099x(arccos713/882)/100
2、圆心角余弦值为(640^2+640^2-355^2)/2x640^2=138633/163840
弧长为3.14x640x(arccos138633/163840)x180=2512x(arccos138633/163840)/225 (以上圆周率取3.14)
参考资料:高一下册数学课本