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Step-by-step explanation:
The answer above is completely correct, but it may help the student to see how to reach it:
We seek a square root of 3−2√2. This must have the form a+b√2.
(a+b√2)2=a2+2ab√2+2b2
So
a2+2b2=3
and
2ab=−2⇒ab=−1
These are two simultaneous equations for a and b.
Rearrange the second:
b=−1a
Substitute into the first:
a2+2a2=3
a4+2=3a2
a4−3a2+2=0
Note that this is a quadratic in a2:
(a2)2−3(a2)+2=0
Factorise:
(a2−2)(a2−1)=0
This gives us two possible solutions for a2: 2 and 1, and so the four solutions for a: ±√2 and ±1.
We are looking for integer solutions for a, and so ±1 are possible solutions. But the other two are possible too - they can simply be folded in to the √2 term. This wouldn't have been possible if we'd had the root of some other number in the solution for a, but this solution is a special case.
Now use the second equation to deduce the four equivalent solutions for b:
b=−1a
b=¯¯¯¯¯+1√2=¯¯¯¯¯+12√2 and ¯¯¯¯+1.
So we have the four solution pairs (a,b):
(√2,−12√2)
(−√2,12√2)
(1,−1)
(−1,1)
This is a bit suspicious - we expect only two solutions, positive and negative square roots, so we wonder if some of these are identical to each other: When we substitute them in to our desired expression a+b√2, we get:
√2−12√2√2=−1+√2
−√2+12√2√2=1−√
So the two solutions with √2 are identical to the two simpler solutions, so we can get rid of them. We now have two solutions, positive and negative square roots:
1−√2
−1+√2
When we take the written square root of a quantity, it is implied that the desired root is the positive root, the "principal value" of the square root function. So we take the single solution that comes from a=+1:
1−√2
Double check: Make sure that this produces the desired answer:
(1−√2)2=1−2√2+2=3−2√2
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