Try for fun, Solve for x where (x^3)-15x=126

Question

anonymous · Answer

Pl dont post the solution. Post  result only OR msg me.

anonymous · Answer

Posting for posterity. Would like to see a hand-done solution.

amistre64 · Answer

id like to see a toe done solution meself :)

amistre64 · Answer

3x^2-15 = 0 when x = +-sqrt(15/3)

6x = 0 when x=0 , so this changes concavity at x=0

we have no undefined points

id say it has to be when x is greater than sqrt(15/3) so far :)

anonymous · Answer

I wonder if you can take the mean distance between change points in concavity, and find the $x$ that way. That's what I tried, but I have a headache and math seems to be difficult right now. :c

shubhamsrg · Answer

x^3 - 15x =126
=> x^3 - 15 x - 126 =0
=>x^3 + 6x^2 + 21x - 6x^2 - 36x - 126=0
should be easier now..

shubhamsrg · Answer

another way can be rational root theorem..

ajprincess · Answer

x=6
x=(+/-sqrt-12)-3=(+/-)(2*sqrt-3)-3

anonymous · Answer

i got x = 6 by noticing that if you add 90 = 15(6)  to 126 you get a lovely cube :)