What I would do is I’ll do that times the big bracket and that times the big bracket so if I separate it I’ll get negative a times a plus b plus b times a plus b and just expand it guys,
negative a times a is negative a squared, negative a times b is negative ab again be careful with the signs negative positive makes a negative okay and then b times a is ab and b times b is b squared and have a look guys we’ve got like terms again we’ve got ab here and ab here they are common aren’t they but what is negative ab plus ab what’s negative one plus one? Zero
So negative ab plus ab is zero ab which is just zero so they actually just cancel out we can just like put a cross through them so we just have negative a squared plus b squared so we do a times four minus a, four times four minus a, it’s the same thing so a times four minus a plus four times 4 minus a expand it out a times 4 is 4a that one is a squared but negative 4 times 4 is 16, 4 times negative a is negative 4a any like terms you can see?
We’ve got a 4a and a negative 4a here they both have a so those are like terms and we don’t have any other like terms what’s four minus four? Zero so we have zero a which is just zero okay so they cancel out so we just have negative a squared plus 16 left okay I really like it when they cancel out don’t they.
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