You can know a perfect square trinomial:
i) if the coefficient of a² = 1.
ii) If you divide the middle number coefficient by 2 and you square it you get the last term.
Take for example the first option:
For all the options, the coefficient of a² = 1
a² + 4a + 16.
Coefficient of a = 4.
4/2 = 2
2² = 4, this does not equal the last term so it is not a perfect square trinomial.
a² + 14a + 49.
Coefficient of a = 14.
14/2 = 7
7² = 49, this is equal the last term so it is a perfect square trinomial.
And the perfect square is (a +7)²
Similarly if you test the last option.
a² + 26a + 169.
Coefficient of a = 26.
26/2 = 13
13² = 169, this is equal the last term so it is a perfect square trinomial.
And the perfect square is (a +13)²
So the only two options are: a² + 14a + 49 and a² + 26a + 169.
Other options do not pass this test.
