Let us denote bn=an for all n. The given inequality is:
(an+1)2+anan+2≤an+an+2
for all n≥1.
We can rearrange the inequality:
(an+1)2+anan+2−an−an+2≤0
(an+1)2−an+anan+2−an+2≤0
(an+1)2−an+(an−1)an+2≤0
Let us try to bound an from above. Suppose an>1 for some n. Since all an>0, we can divide both sides by an (if needed), but let's try to use the structure.
Suppose an≤1 for all n. Then a2022≤1 as desired. Suppose not, and let k be the smallest index such that ak>1.
Since a1>0, and a1≤1 (otherwise k=1), so a1≤1, a2≤1, ..., ak−1≤1, ak>1.
Now, consider the inequality for n=k−1:
(ak)2+ak−1ak+1≤ak−1+ak+1
(ak)2≤ak+1(1−ak−1)+ak−1
But ak−1≤1, so 1−ak−1≥0, and ak+1>0.
So ak+1(1−ak−1)+ak−1≤ak+1+ak−1.
But (ak)2≤ak+1(1−ak−1)+ak−1≤ak+1+ak−1.
But ak>1, so (ak)2>1.
Therefore, ak+1(1−ak−1)+ak−1>1.
But ak−1≤1, so ak+1(1−ak−1)>1−ak−1≥0.
So ak+1>1 unless ak−1=1.
If ak−1=1, then ak+1(1−ak−1)=0, so (ak)2≤1.
But ak>1, so (ak)2>1, contradiction.
Therefore, ak−1<1, so 1−ak−1>0, and ak+1>1−ak−11−ak−1=1.
So ak+1>1.
Thus, if ak>1 for some k, then ak+1>1 as well. But then, by induction, an>1 for all n≥k.
Now, consider the original inequality for n≥k:
(an+1)2+anan+2≤an+an+2
But an>1, an+2>1, so an+an+2<(an)(an+2) (since an,an+2>1).
So anan+2>an+an+2.
Therefore,
(an+1)2+anan+2>(an+1)2+an+an+2
But the inequality says (an+1)2+anan+2≤an+an+2, so (an+1)2≤0, which is impossible since an+1>0.
Therefore, our assumption that ak>1 for some k leads to a contradiction. Thus, an≤1 for all n, in particular a2022≤1.