Упростить выражения.а) 3(а+с)(в квадрате)-6ас=б) (с-2)(с+3)-(с-1)(в...

Решите задачу:

$3(a+c)^{2}-6ac=3(a^{2}+2ac+c^{2})-6ac=3a^{2}+6ac+3c^{2}-6ac= \\ =3a^{2}+3c^{2}=3(a^{2}+c^{2})$
$(c-2)(c+3)-(c-1)^{2}=c^{2}-2c+3c-6-(c^{2}-2c+1)= \\ =c^{2}-2c+3c-6-c^{2}+2c-1=3c-7$
$2x(x-3)-3x(x+5)=2x^{2}-6x-3x^{2}-15x=-x^{2}-21x$
$x^{3}-9x=x(x^{2}-9)=x(x-3)(x+3) \\ -5a^{2}-10ab-5b=-5(a^{2}+2ab+b^{2})=-5(a+b)^{2}= \\ -5(a+b)(a+b) \\ 81a^{4}-1=(9a^{2})^{2}-1=(9a^{2}-1)(9a^{2}+1)=((3a)^{2}-1)(9a^{2}+1)= \\ (3a-1)(3a+1)(9a^{2}+1)$