Use three four five six seven eight six numbers to form two three digit numbers, make the product of these two three digit numbers maximum, how to arrange, and explain the thinking

Use three four five six seven eight six numbers to form two three digit numbers, make the product of these two three digit numbers maximum, how to arrange, and explain the thinking

843*765
If the value of a + B is fixed, when the absolute value of A-B is minimum, the product is maximum
Let a + B = K
ab=a(k-a)=-a^2+ak=-(a-k/2)^2+(k/2)^2
When the absolute value of A-K / 2 is minimum, AB has extremum

Nine of ten sequences of two addition formulas, each number used once, and equal

There is no correct answer, because the sum of ten numbers 0 - 9 is 45 divided by 2

Does the general term formula an = (n + 1) * 0.9 ^ n of the sequence an have the natural number n of the term, so that for any natural number n there is an

q=a(n+1)/a(n)=[(n+2)*0.9^(n+1)]/[(n+1)*0.9^n]=
=9(n+2)/10(n+1),
When N1, a (n + 1) > A (n);
When n = 8, 9 (n + 2) / 10 (n + 1) = 1, a (8) = a (9);
When n > 8, 9 (n + 2) / 10 (n + 1)