7a/6c -(49a²+36c²)/42ac+(6c-49a)/7a=
=(49a²-49a²-36c²+36c²-6c*49a)/42ac=-6c*49a/42ac=-7
применим формулу разность квадратов
(a+b)(a -b)= a²- b²
(a³- b³)(a³+b³<span>)(a</span>⁶+b⁶)= (a⁶-b⁶)(a⁶+b⁶)= (a¹² -b¹²)
X²(x³-1)+(x²-1)=x²(x-1)(x²+x+1)+(x-1)(x+1)=
=(x-1)(x^4+x³+x²+x+1)