= 15x+ 12-3x+5

= 12x+ 17

8a+2b(5a-b)

=8a+ 10ab-2b？

(2) 2x？ -mnx？ +y？ , which is a cubic trinomial about the letters x and y.

This is not right, it should be once every three times.

There are three terms, so the coefficient here is not equal to

So mn≠0

So m≠0, n≠0.

(3) a? x？ +ax？ -4x？ +2x？ +x+ 1

=(a？ -4)x？ +(a+2)x？ +x+ 1

It's a quadratic trinomial about x.

So the maximum x is 2 times.

So the cubic coefficient is 0.

So a? -4=0

Answer? =4

a= 2

Meanwhile, the quadratic coefficient is not equal to 0.

a+2≠0

a≦-2

So a=2

So a? +a=8+2= 10