Example1. Expand `(2x-1)(4x+5)`
Solution:
`(2x-1)(4x+5)`
FOIL method : `(a+b)(c+d)=ac+ad+bc+bd`
`=(color{red}{2x}+color{blue}{(-1)})(color{green}{4x}+color{purple}{5})`
`=color{red}{2x}*(color{green}{4x}+color{purple}{5})+color{blue}{(-1)}*(color{green}{4x}+color{purple}{5})`
`=color{red}{2x}*color{green}{4x}+color{red}{2x}*color{purple}{5}+color{blue}{(-1)}*color{green}{4x}+color{blue}{(-1)}*color{purple}{5}`
`=8x^2+10x+(-4x)+(-5)`
`=8x^2+6x-5`
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