We have,
a2−b24sin−1a−b√a2−b2
⇒a2−b24sin−1a−b√(a+b)(a−b)
⇒a2−b24sin−1√a−b√a−b√a+b√a−b
⇒a2−b24sin−1√a−b√a+b
Hence, this is the answer.