Упростите выражение (№115,116,121,122) Срочно!

115.
$\frac{2a+2b}{b} * (\frac{1}{a-b} - \frac{1}{a+b})= \frac{2(a+b)}{b} *( \frac{a+b-a+b}{(a+b)(a-b)} ) = \frac{2*2b}{b(a-b)} = \frac{4}{a-b}$

116.
$( \frac{1}{m-n} - \frac{1}{m+n} ): \frac{2}{3m-3n} = \frac{m+n-m+n}{(m+n)(m-n)} * \frac{3(m-n)}{2} = \frac{2*3n}{2(m+n)} = \frac{3n}{m+n}$

121.
$( \frac{1}{a} - \frac{1}{b})* \frac{2ab}{ a^{2} - b^{2} } = \frac{b-a}{ab} * \frac{2ab}{(a-b)(a+b)} =- \frac{a+b}{ab}* \frac{2ab}{(a-b)(a+b)} =- \frac{2}{a-b}$

122.
$( \frac{1}{a} - \frac{1}{b} ): \frac{b^{2}- a^{2}}{ab^{2} } = \frac{b-a}{ab} * \frac{a b^{2} }{(b-a)(b+a)} = \frac{b}{b+a}$