(a+b)2

four

-ab=

a2+2ab+b2-4ab

four

=

a2-2ab+b2

four

=

(A-B) 2

four

≥0, so ab≤

(a+b)2

four

Established.

(2) If the function f(x)=x2-ax+ 1 has no zero, the discriminant △ < 0 of the corresponding equation, that is, A2-4 < 0, is solved as -2 < a < 2. Therefore, A < 2 "is a necessary condition for the function f(x)=x2-ax+ 1 to have no zero.

All wet.

③ When x0=-

1

2

X02+x0 < 0 holds, so ③ is correct.

The inverse proposition of the proposition "If the last digit of an integer is 0, then this integer can be divisible by 5" is "If an integer can be divisible by 5, then the last digit of this integer is 0", which is wrong because the last digit may be 5.

So choose a.