x-y=2a - ②
Firstly, ①+② gives x =1+a.
Then ①+② gives y =1-a.
And because x>0, y<0
That is,1+a >; 0, 1-a & lt; 0
Get a>- 1, a> 1
So in general, you can get a> 1
2、x+5 & gt; 0
x & gt-5
x+a & lt; 0
X<[ [ancient names or Latin modern names of animals and plants]
So-5
If there is no solution, -5≥-a
So a≥5
3. From 5x+3y=3 1.
x+y-p=0
Get 2x=3 1-3p.
X=(3 1-3p)/2 is
positive integer
Get 2y=
5p
-3 1
Y=(5p-3 1)/2 is a positive integer.
So 3 1-2p≥2.
5p-3 1≥2
Solution p≤29/2
p≥33/5
So p = 7, 8, 9, 10,1,12, 13, 14.
Because x and y are positive integers.
So p = 7,9