Solve 1/216{left(1-sqrt{17}right)}^3+1/216{left(1+sqrt{17}right)}^3

$$\frac{ 1 }{ 216 } { \left(1- \sqrt{ 17 } \right) }^{ 3 } + \frac{ 1 }{ 216 } { \left(1+ \sqrt{ 17 } \right) }^{ 3 }$$

Evaluate

$$\frac{13}{27}\approx 0.481481481$$

Solution Steps

Use binomial theorem \left(a-b\right)^{3}=a^{3}-3a^{2}b+3ab^{2}-b^{3} to expand \left(1-\sqrt{17}\right)^{3}.

$$\frac{1}{216}\left(1-3\sqrt{17}+3\left(\sqrt{17}\right)^{2}-\left(\sqrt{17}\right)^{3}\right)+\frac{1}{216}\left(1+\sqrt{17}\right)^{3}$$

The square of \sqrt{17} is 17.

$$\frac{1}{216}\left(1-3\sqrt{17}+3\times 17-\left(\sqrt{17}\right)^{3}\right)+\frac{1}{216}\left(1+\sqrt{17}\right)^{3}$$

Multiply 3 and 17 to get 51.

$$\frac{1}{216}\left(1-3\sqrt{17}+51-\left(\sqrt{17}\right)^{3}\right)+\frac{1}{216}\left(1+\sqrt{17}\right)^{3}$$

Add 1 and 51 to get 52.

$$\frac{1}{216}\left(52-3\sqrt{17}-\left(\sqrt{17}\right)^{3}\right)+\frac{1}{216}\left(1+\sqrt{17}\right)^{3}$$

Use the distributive property to multiply \frac{1}{216} by 52-3\sqrt{17}-\left(\sqrt{17}\right)^{3}.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(1+\sqrt{17}\right)^{3}$$

Use binomial theorem \left(a+b\right)^{3}=a^{3}+3a^{2}b+3ab^{2}+b^{3} to expand \left(1+\sqrt{17}\right)^{3}.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(1+3\sqrt{17}+3\left(\sqrt{17}\right)^{2}+\left(\sqrt{17}\right)^{3}\right)$$

The square of \sqrt{17} is 17.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(1+3\sqrt{17}+3\times 17+\left(\sqrt{17}\right)^{3}\right)$$

Multiply 3 and 17 to get 51.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(1+3\sqrt{17}+51+\left(\sqrt{17}\right)^{3}\right)$$

Add 1 and 51 to get 52.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(52+3\sqrt{17}+\left(\sqrt{17}\right)^{3}\right)$$

Use the distributive property to multiply \frac{1}{216} by 52+3\sqrt{17}+\left(\sqrt{17}\right)^{3}.

$$\frac{13}{54}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{13}{54}+\frac{1}{72}\sqrt{17}+\frac{1}{216}\left(\sqrt{17}\right)^{3}$$

Add \frac{13}{54} and \frac{13}{54} to get \frac{13}{27}.

$$\frac{13}{27}-\frac{1}{72}\sqrt{17}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{72}\sqrt{17}+\frac{1}{216}\left(\sqrt{17}\right)^{3}$$

Combine -\frac{1}{72}\sqrt{17} and \frac{1}{72}\sqrt{17} to get 0.

$$\frac{13}{27}-\frac{1}{216}\left(\sqrt{17}\right)^{3}+\frac{1}{216}\left(\sqrt{17}\right)^{3}$$

Combine -\frac{1}{216}\left(\sqrt{17}\right)^{3} and \frac{1}{216}\left(\sqrt{17}\right)^{3} to get 0.

$$\frac{13}{27}$$

Factor

$$\frac{13}{3 ^ {3}} = 0.48148148148148145$$