The square of (a+b), also written as (a+b)², is:

a² + 2ab + b²

This can be derived as follows:

• (a+b)² = (a+b) * (a+b)
• = a*(a+b) + b*(a+b)
• = a*a + a*b + b*a + b*b
• = a² + ab + ba + b²
• = a² + ab + ab + b² (since ab = ba)
• = a² + 2ab + b²

This is a fundamental algebraic identity.