Factor completely 4a^2-81?

4a^2-81 factors to (2a-9)(2a+9)

4a^2-81 is a perfect square
and the special formula for perfect squares is
(a+b)(a-b)
so it would give you
[(2a+9)(2a-9)] =answer
and to check just multiply it out
which gives you
4a^2+9a-9a-81
then combine like terms and you get
4a^2-81
so now you know your answer is right

4x²-81
this is a difference of squares

(2a -9)(2a+9)

hope this helps!!!

(2a - 9) (2a + 9)

4a^2 -81=0
(2a+9) (2a-9)..

We know general formula x^2-y^2=(x+y)(x-y)
4a^2-81=(2a)^2-(9)^2
Here x=2a and y=9 .So, applying the formula
4a^2-81=(2a)^2-(9)^2
=(2a+9)(2a-9)

a^2 - b^2 = (a + b)(a - b)

4a^2 - 81
= (2a + 9)(2a - 9)