Identify the trinomial that is a perfect square. 81x^2 - 70x 16 64x^2 - 90x 25 81x^2 - 72x 16 64x^2 - 70x 25

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