There are some binomials that can be expanded directly by a general inspection. For example (x+a)(x+b)=x^2+x(a+b)+ab.

Can you help me to recognize to which case does the following expression belongs to?

(3x+7)(7x-3)

Question

There are some binomials that can be expanded directly by a general inspection. For example (x+a)(x+b)=x^2+x(a+b)+ab.

Can you help me to recognize to which case does the following expression belongs to?

(3x+7)(7x-3)

anonymous · Answer

I found the answer guys :)

radar · Answer

Did you expand it by inspection?  Or did you use paper & pencil?

anonymous · Answer

after a while, FOILing with small numbers becomes easy

in the example above, you would do
I'm not sure where i learned this from, or maybe i came up with it (i'm definitely not the first to do so!)

(ax + b)(cx + d)
(a*b)x^2 + (bc + ad)x + bd

it's really just a "formula" for FOIL

so your example
(3x+7)(7x-3)
(3 * 7)x^2 + (49 + -9)x + -21
21x^2 + 40x - 21