EASY MATH QUESTION!!!

I do not know how to factorise this:

25x^4 - 49y^2

You need to think of the FOIL method--but the two different parenthesis have to be opposite so that it cancels out.

Start with 25x^4 -- 5x^2 x 5x^2=25x^4.
Then, 49y^2 -- 7y x 7y=49y^2.

Since there is a - and not a +, one of the 7y's has to be negative. This will also cancel out the other number (if you do (5x^2 +7y) (5x^2+7y) you will get 25x^4+70yx^2+49y^2). So, you know that the answer has to be:

(5x^2 + 7y)(5x^2 - 7y). A way to check your answer is to FOIL (First, Outer, Inner, Last)

FIRST: 5x^2 x 5x^2=25x^4
OUTER: 5x^2 x -7y=-35yx^2
INNER: 5x^2 x 7y= 35yx^2
LAST: 7y x -7y = -49y^2

25x^4-35yx^2+35yx^2-49y^2 =25x^4-49y^2

As everyone said, this is a difference of two squares questions, therefore the answer would be
(5x^2 - 7y)^2
= (5x^2-7y)*(5x^2+7y)
[Using FOIL (First, Outer, Inner, Last)]=25x^4+35x^2y-35x^2y-49y^2
[simplifying]=25x^4-49y^2

This is called the difference of two squares:

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

So your answer is:

(5x^2-7y)(5x^2 + 7y)

hope this helps.

(5x^2 + 7y)(5x^2 - 7y)