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Question Number 51772 by Cheyboy last updated on 30/Dec/18

x^3 +12x+12=0    Express your answer in surd form

$$\mathrm{x}^{\mathrm{3}} +\mathrm{12x}+\mathrm{12}=\mathrm{0} \\ $$$$ \\ $$$$\mathrm{Express}\:\mathrm{your}\:\mathrm{answer}\:\mathrm{in}\:\mathrm{surd}\:\mathrm{form} \\ $$$$ \\ $$

Answered by ajfour last updated on 30/Dec/18

x=(−6+(√(36+64)))^(1/3) −(6+(√(36+64)))^(1/3)       = (4)^(1/3) −((16))^(1/3)  .

$${x}=\left(−\mathrm{6}+\sqrt{\mathrm{36}+\mathrm{64}}\right)^{\mathrm{1}/\mathrm{3}} −\left(\mathrm{6}+\sqrt{\mathrm{36}+\mathrm{64}}\right)^{\mathrm{1}/\mathrm{3}} \\ $$$$\:\:\:\:=\:\sqrt[{\mathrm{3}}]{\mathrm{4}}−\sqrt[{\mathrm{3}}]{\mathrm{16}}\:.\: \\ $$

Commented by Cheyboy last updated on 30/Dec/18

Thank you sir. but i would appreciate  to know the working

$$\mathrm{Thank}\:\mathrm{you}\:\mathrm{sir}.\:\mathrm{but}\:\mathrm{i}\:\mathrm{would}\:\mathrm{appreciate} \\ $$$$\mathrm{to}\:\mathrm{know}\:\mathrm{the}\:\mathrm{working} \\ $$

Answered by mr W last updated on 31/Dec/18

x^3 +12x+12=0  cubic eqn. of this kind can be solved  in following way:    we know that  u^3 +v^3 =(u+v)(u^2 −uv+v^2 )=(u+v)[(u+v)^2 −3uv]  ⇒ (u+v)^3 −3uv(u+v)−(u^3 +v^3 )=0    when you compare this with our eqn.  you can see that   if we put 3uv=−12, u^3 +v^3 =−12,  then u+v is the solution of our eqn.    3uv=−12⇒v=−(4/u)  u^3 +v^3 =−12⇒u^3 −(4^3 /u^3 )+12=0  ⇒u^6 +12u^3 −64=0   (quadratic eqn. for u^3 )  ⇒(u^3 +16)(u^3 −4)=0  ⇒u^3 = 4   (−16 will also do, but gives the same)  ⇒u=(4)^(1/3)   ⇒v=−(4/(4)^(1/3) )=−2(2)^(1/3)   ⇒u+v=(4)^(1/3) −2(2)^(1/3)     i.e. the solution of x^3 +12x+12=0 is  x=(4)^(1/3) −2(2)^(1/3) ≈−0.9324

$${x}^{\mathrm{3}} +\mathrm{12}{x}+\mathrm{12}=\mathrm{0} \\ $$$${cubic}\:{eqn}.\:{of}\:{this}\:{kind}\:{can}\:{be}\:{solved} \\ $$$${in}\:{following}\:{way}: \\ $$$$ \\ $$$${we}\:{know}\:{that} \\ $$$${u}^{\mathrm{3}} +{v}^{\mathrm{3}} =\left({u}+{v}\right)\left({u}^{\mathrm{2}} −{uv}+{v}^{\mathrm{2}} \right)=\left({u}+{v}\right)\left[\left({u}+{v}\right)^{\mathrm{2}} −\mathrm{3}{uv}\right] \\ $$$$\Rightarrow\:\left({u}+{v}\right)^{\mathrm{3}} −\mathrm{3}{uv}\left({u}+{v}\right)−\left({u}^{\mathrm{3}} +{v}^{\mathrm{3}} \right)=\mathrm{0} \\ $$$$ \\ $$$${when}\:{you}\:{compare}\:{this}\:{with}\:{our}\:{eqn}. \\ $$$${you}\:{can}\:{see}\:{that}\: \\ $$$${if}\:{we}\:{put}\:\mathrm{3}{uv}=−\mathrm{12},\:{u}^{\mathrm{3}} +{v}^{\mathrm{3}} =−\mathrm{12}, \\ $$$${then}\:{u}+{v}\:{is}\:{the}\:{solution}\:{of}\:{our}\:{eqn}. \\ $$$$ \\ $$$$\mathrm{3}{uv}=−\mathrm{12}\Rightarrow{v}=−\frac{\mathrm{4}}{{u}} \\ $$$${u}^{\mathrm{3}} +{v}^{\mathrm{3}} =−\mathrm{12}\Rightarrow{u}^{\mathrm{3}} −\frac{\mathrm{4}^{\mathrm{3}} }{{u}^{\mathrm{3}} }+\mathrm{12}=\mathrm{0} \\ $$$$\Rightarrow{u}^{\mathrm{6}} +\mathrm{12}{u}^{\mathrm{3}} −\mathrm{64}=\mathrm{0}\:\:\:\left({quadratic}\:{eqn}.\:{for}\:{u}^{\mathrm{3}} \right) \\ $$$$\Rightarrow\left({u}^{\mathrm{3}} +\mathrm{16}\right)\left({u}^{\mathrm{3}} −\mathrm{4}\right)=\mathrm{0} \\ $$$$\Rightarrow{u}^{\mathrm{3}} =\:\mathrm{4}\:\:\:\left(−\mathrm{16}\:{will}\:{also}\:{do},\:{but}\:{gives}\:{the}\:{same}\right) \\ $$$$\Rightarrow{u}=\sqrt[{\mathrm{3}}]{\mathrm{4}} \\ $$$$\Rightarrow{v}=−\frac{\mathrm{4}}{\sqrt[{\mathrm{3}}]{\mathrm{4}}}=−\mathrm{2}\sqrt[{\mathrm{3}}]{\mathrm{2}} \\ $$$$\Rightarrow{u}+{v}=\sqrt[{\mathrm{3}}]{\mathrm{4}}−\mathrm{2}\sqrt[{\mathrm{3}}]{\mathrm{2}} \\ $$$$ \\ $$$${i}.{e}.\:{the}\:{solution}\:{of}\:{x}^{\mathrm{3}} +\mathrm{12}{x}+\mathrm{12}=\mathrm{0}\:{is} \\ $$$${x}=\sqrt[{\mathrm{3}}]{\mathrm{4}}−\mathrm{2}\sqrt[{\mathrm{3}}]{\mathrm{2}}\approx−\mathrm{0}.\mathrm{9324} \\ $$

Commented by Cheyboy last updated on 30/Dec/18

Thank you very much sir

$$\mathrm{Thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}\:\mathrm{sir} \\ $$$$ \\ $$

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