25. Given in $\triangle A B C$ that $a$, $b$, and $c$ are the three sides of the triangle, and $a: b: c=20: 29: 21$.
Find: (1) the cosine of the largest angle of $\triangle A B C$;
(2) the sine of the angle opposite side $a$;
(3) the tangent of the angle opposite side $c$.