若mn=a,1/m²+1/n²=b,则(m+n)²=?

mn=a,1/m²+1/n²=b
(1/m +1/n)² = 1/m² + 2/mn + 1/n² = b + 2/a
但是 (1/m+1/n)² = [(m+n)/mn]² = (m+n)² / m²n² = (m+n)²/a²
所以 b + 2/a = (m+n)²/a²
(m+n)² = a²b +2a