21. а 2 а–2
----------- – ----------- = ----------- =
(а–2)^2 (а–2)^2 (а–2)^2
1
= -------
а–2
23. b(a+b)•a ab
--------------- = ------
(a–b)(a+b) a–b
найдем производную, она равна -9/x^2+1=(x^2-9)/x^2=(x-3)(x+3)/x^2
√2 * √8 = √(2 * 8) = √16 = 4,
√2 / √18 = √(2/18) = √(1/9) = 1/3,
√27 * √3 = √(27 * 3) = √81 = 9,
√52 / √117 = √(52/117) = √(4/9) = 2/3