Therefore, the lengths of the three sides are: 3×5= 15, 4×5=20, 5× 5 = 25;

(2) Prove that all three sides are integer multiples of 3, 4 and 5,

If it is set to k times, then the three sides are 3k, 4k and 5k respectively.

A triangle is a right triangle, and 3k and 4k are right sides.

Its area is S= (3k)? (4k)=6k2，

∴k2=, k= (positive value),

That is, divide the area by 6, and then find the root to get the multiple.