hmm
(a+b)^2=(a+b)(a+b)=a^2+2ab+b^2
basically
for
ax^2+bx+c
it is a perfect trinomial when
b=2(√a)(√c)
remember to take both positive and negative roots into consideration
because
(a+b)^2=a^2+2ab+b^2 and
(a-b)^2=a^2-2ab+b^2
see each
first one
-70=2(9)(4)
-70=72
false
second
-90=2(8)(5)
-90=80
false
third
-72=(9)(4)
-72=72
false
but, the last one could be negative
(9x-4)^2 is factor
that is the answer
the answer is 81x^2-72x+16
