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I wanna know what x^2 is equal to if
(x+9)^(1/3) - (x-9)^(1/3) = 3
I've got this, but I'm not sure ..someone tell me if I'm using magic anywhere.
[(x + 9)^(1/3)]^3 = [3 + (x - 9)^(1/3)]^3 //cube both sides
x + 9 = 27 + x - 9
(x + 9)^2 = (27 + x - 9)^2 //square both sides
x^2 + 18x + 81 = x^2 + 36x + 324 //the x^2 cancels out
-243 = 18x
x = -243/18 //solve for x
so if you square both sides again...
x^2 = (-243/18)^2
well, my calculator seems to dissagree, I'm trying to work out why though.
edit:
OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
huh...i figured you have to cube it if you wanna get rid of the radicals and all. :\Quote:
Originally posted by Acidic
well, my calculator seems to dissagree, I'm trying to work out why though.
edit:
OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
your expansion of [3 + (x - 9)^(1/3)]^3 is incorrect
[3 + (x - 9)^(1/3)]^3 <> 27 + x - 9
omg...i see it now. thank you so much.Quote:
Originally posted by da_silvy
your expansion of [3 + (x - 9)^(1/3)]^3 is incorrect
[3 + (x - 9)^(1/3)]^3 <> 27 + x - 9
i reworked it w/the correct cubic expansion...and i got x=8.94427 so x^2 = 8.94427^2 ..whatever that isQuote:
Originally posted by Acidic
well, my calculator seems to dissagree, I'm trying to work out why though.
edit:
OK, I'm pretty sure that 8.9447 is the right answer (though I don't know how to work that out without a calculator).
I'mpretty sure it's the cubing everything that seems to upset things, but I have no clue how else I could do this.
i got x^2 = 80
same as you
Quote:
Originally posted by da_silvy
i got x^2 = 80
same as you
good show
you guys know an easier way to do this? cuz the cubic expansion is ugly as you know what..
That made my computer think for a bit....
x^2=80 though
or x=8.944