Which type of proof is best for this problem?

Let $()=1+2+...+$ be the sum of the first $$ natural numbers and $()=13+23++3$ be the sum of the first $$ cubes. Prove that $()=2()$ for every $$ and that the following is true:

$()=12(+1)$

$()=142(+1)2$

a$.$

proof by construction

b$.$

proof by induction

c$.$

proof by contradiction