Given:
The given expression is

Find:
we have to check whether the given expression is a perfect square trinomials or not.
Step-by-step explanation:
An expression obtained from the square of a binomial equation is a perfect square trinomial. An expression is said to a perfect square trinomial if it takes the form ax^2 + bx + c and satisfies the condition b^2 = 4ac.
compare x^2-20x+50 with ax^2 + bx + c, we have
a=1, b = -20, c =50
Now

Therefore, b^2 is not equal to 4ac,
Hence the given expression is not a perfect square trinomials.