- #1
Govind_Balaji
- 83
- 0
Homework Statement
$$\sqrt{2}+\sqrt{7}-\sqrt{10}$$ is a surd
Find another irrational number such that when multiplied by $$\sqrt{2}+\sqrt{7}-\sqrt{10}$$ , it results a rationall number.
Homework Equations
The Attempt at a Solution
I found the answer, but I think I did not find in the correct method.
My first attempt:
I tried multiplying $$\sqrt{2}+\sqrt{7}-\sqrt{10} with \sqrt{2}+\sqrt{7}-\sqrt{10}.$$
I got
[itex]
\\
\left( \sqrt{2}+\sqrt{7}-\sqrt{10} \right )\left (\sqrt{2}+\sqrt{7}-\sqrt{10} \right )\\
=2+\sqrt{14}-\sqrt{20}+\sqrt{14}+7-\sqrt{70}-\sqrt{20}-\sqrt{70}+10\\
=19+2\sqrt{14}-2\sqrt{20}-2\sqrt{70}
[/itex]
I didn't get a rational number
My second attempt:
So I tried,
[itex]
\\
\left (\sqrt{2}+\sqrt{7}-\sqrt{10} \right )\left ( \sqrt{2}+\sqrt{7}+\sqrt{10} \right )\\
=2+\sqrt{14}+\sqrt{20}+\sqrt{14}+7+\sqrt{70}-\sqrt{20}-\sqrt{70}-10\\
=2\sqrt{14}-1\\
[/itex]
Extension of my second attempt
I thought I could multiply further.
[itex]
\\
\left (2\sqrt{14}-1 \right )\left (2\sqrt{14}+1\right )\\
=4*14-1\\
=56-1=55\\
[/itex]
I got a rational number by multiplying [itex]\left ( \sqrt{2}+\sqrt{7}+\sqrt{10} \right )[/itex] with
[itex]\left (\sqrt{2}+\sqrt{7}-\sqrt{10} \right )[/itex] and further multiplying with [itex]\left (2\sqrt{14}+1\right )[/itex]
That is, multiplying factor=
[itex]\left ( \sqrt{2}+\sqrt{7}+\sqrt{10} \right )\left (2\sqrt{14}+1\right )[/itex]
I think I got the answer with some guess work in my second attempt. In its extension I used
$$a^2-b^2$$ identity.
Is there any identity or a logical way to solve without guessing?