Which polynomial is a perfect square trinomial?

Question

goformit100 · Answer

ax^3+by^2-cy

anonymous · Answer

the answers are 
36x2 − 18x − 9 

 4a2 − 28a − 49 

 25b2 − 40b + 16 

 4x2 − 6x + 9

anonymous · Answer

if any of those choices can be factored in this form  (M $\pm N)^2 $ where M and N are expressions then they are perfect square trinomials...:)

anonymous · Answer

for those answers

36x^2 - 18x - 9
9(4x^2 - 2x - 1)
9(2x-1)(2x-1)
9(2x-1)^2
factors out a 9 so not perfect

4a^2 - 28a - 49
not perfect because you need 49 to be positive for it to factor

25b^2 - 40b + 16
(5b-4)(5b-4)
(5b - 4)^2
so this is a perfect square! C is correct

but jic, check the last one
4x^2 - 6x + 9
can't factor, not perfect because it would only be perfect if the b term was -12, not -6

so the third answer is correct