Question

1. m(m-3)(m+3)-(m-2)(m²+2m+4)при m= -⅔ 2. (6m - n)(6m+n)-(12m-5n)(3m+n) при m = - 8/9,n = ¾.
блин я тупая как пробка(

Answer 1

$1) \ m(m-3)(m+3)-(m-2)(m^{2}+2m+4)=m(m^{2}-9)-(m^{3}-8)=\\\\=m^{3}-9m-m^{3}+8=8-9m\\\\m=-\dfrac{2}{3} \ \Rightarrow \ 8-9m=8-9\cdot\Big(-\dfrac{2}{3}\Big)=8+3\cdot2=\boxed{14} \\\\\\2) \ (6m-n)(6m+n)-(12m-5n)(3m+n)=\\\\(6m)^{2}-n^{2}-12m\cdot3m-12m\cdot n+5n\cdot3m+5n\cdot n= \\\\36m^{2}-n^{2}-36m^{2} -12mn+15mn+5n^{2}=4n^{2}+3mn\\\\m=-\dfrac{8}{9} \ ; \ n=\dfrac{3}{4}$

$4n^{2} +3mn=4\cdot\Big(\dfrac{3}{4}\Big )^{2}+3\cdot\Big(-\dfrac{8}{9}\Big)\cdot\Big(\dfrac{3}{4}\Big)=\dfrac{9}{4} -\dfrac{8}{4}=\dfrac{1}{4}=\boxed{0,25}$