Answer:
(5x +4y)^2
Explanation:
The first and last terms are both perfect squares, and the middle term is twice the product of their roots. That means the trinomial is the perfect square trinomial ...
25x^2 +40xy +16y^2 = (5x +4y)^2
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It matches the pattern ...
a^2 +2ab +b^2 = (a +b)^2