Hi everyone! Can someone help me factor
r^3 + 3r^2 - 4 = 0 please? My book says it is obvious that by inspection one of the roots are r=1...not so obvious to me. Thanks! :o)

Question

Hi everyone! Can someone help me factor 
r^3 + 3r^2 - 4 = 0 please? My book says it is obvious that by inspection one of the roots are r=1...not so obvious to me. Thanks! :o)

anonymous · Answer

$$1+3-4=0$$ is more or less obvious

anonymous · Answer

if $r=1$ you add the coefficients
that is always true

welshfella · Answer

well if you plug in 1 for r you get 1^3 + 3*1^2 - 4 = 0

anonymous · Answer

@satellite...why is it okay to do 1+3-4=0 i mean what about grouping or all the other methods of solving...please explain why it's okay to just look at the coefficients in that way

misty1212 · Answer

HI!!
why does everyone start with "well"
seem rather affected to me
as does "you just..."

anonymous · Answer

1 to any power is 1

anonymous · Answer

so for example if you have 
$$f(x)=4x^3+5x^2+2x-1$$ then 
$$f(1)=4+5+2-1$$

ribhu · Answer

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