5.[3(a+b)^3 -2(a+b)^2-4(a+b)]/(a+b)=3(a+b)^2 -2(a+b)-4=3a^2 +6ab+3b^2-2a-2 b-4

6. The common denominator is 16, namely (1234 *15+2468 *1+617 *14)/16.

7. There is a 2nx missing from the title: I know that the product does not contain quadratic terms and cubic terms, so let the coefficient of quadratic terms be zero. x^2 *(-3x)+mx^2 +2nx^3 +3x^2 =(2n-3)x^3 +(3+m-6n)x^2； So m=6 and n=3/2.

8. Backward deduction: (a+ 1/a) * a = a 2+ 1 = 3a (original formula shifted); Therefore, (a+1/a) = 3;

(a+1/a) 2 = a 2+2+(1/a) 2 = 9, so [a 2+1/(a 2)] = 7.

9. As long as the item containing A is calculated, a-8a+a+6a=0, then the value has nothing to do with A..