2.Сократите дробь:28а^2/21а 3.Упростите дробь:4а^2-12аb+9b^2/2a-3b. Найдите значение...

Решите задачу:

$\frac{28 a^{2} }{21a} = \frac{4a}{3} \\. \\ \\ \frac{4a^{2} -12ab+9b^{2} }{2a-3b} = \frac{(2a-3b)(2a-3b)}{2a-3b} =2a-3b \\ a=2, b=3 \\ 2*2-3*3=-5 \\ . \\ \\ \frac{1}{2a} + \frac{1}{a} = \frac{1+2}{2a} = \frac{3}{2a} \\ . \\ \\ \frac{3}{3-y} - \frac{y}{ y^{2} -9}=\frac{3}{3-y}+ \frac{y}{9- y^{2} } = \frac{3}{3-y} + \frac{y}{(3-y)(3+y)} = \\ \\ \frac{3(3+y)+y}{(3-y)(3+y)} = \frac{9+4y}{9-y^{2} } \\ . \\ \\$ $\frac{5x}{7y} * \frac{14y}{15x} = \frac{2}{3} \\ \\ \frac{ a^{2} -ab}{ b^{2} +ab}: \frac{3a-3b}{6(a+b)} = \frac{a(a-b)}{b(a+b)}* \frac{6(a+b)}{3(a-b)} = \frac{2a}{b} \\ . \\ \\ \frac{ a^{2} -25}{a+3} * \frac{1}{ a^{2} +5a} - \frac{ a^{2} +10a+25}{a^{3} -6a +9a} = \frac{(a+5)(a-5)}{a+3} * \frac{1}{a(a+5)}- \frac{a^{2} +10a+25}{a( a^{2} -6+9)} = \\ \\ \frac{a-5}{a(a+3)} - \frac{a^{2} +10a+25}{a( a^{2} +3)} = \frac{(a-5)( a^{2} +3)-(a^{2} +10a+25)(a+3)}{a(a+3)( a^{2} +3)} = \\ \\$ $\frac{a^{3}+3a-5 a^{2} -15- a^{3}-3a^{2} -10a^{2} -30a-25a-75 }{a(a+3)( a^{2} +3)} = \frac{-18 a^{2} -52a-90}{a(a+3)(a^{2} +3)}$