Question
Find the value of the square root \sqrt{17+12\sqrt{2}}
Find the value of the square root \sqrt{17+12\sqrt{2}}
Let
17+12\sqrt{2} = (a+b\sqrt{2})^2
17-12\sqrt{2} = (a-b\sqrt{2})^2
in which a and b are positive integers
Addition of the two equations gives
(a+b\sqrt{2})^2+(a-b\sqrt{2})^2 = 34
Expand and simplify the equation
2a^2+2\cdot 2b^2=34
Multiplying the two equations gives
(17+12\sqrt{2})(17-12\sqrt{2}) = (a+b\sqrt{2})^2 (a-b\sqrt{2})^2
Simplify the equation using difference of square formula
(a^2 - b^2\cdot 2)^2 = 17^2 - 12^2\cdot 2=1
Taking the square root
2 systems of equations are obtained from (1) and (2)
Case 1
\begin{cases} a^2+2b^2&=17 \\ a^2 - 2b^2 &=1 \end{cases}
Solving the system of equaitons gives
a = \sqrt{\dfrac{17+1}{2}} =\sqrt{9} = 3
b = \sqrt{\dfrac{17-1}{2\cdot 2}}=\sqrt{4}= 2
Case 2
\begin{cases} a^2+2b^2&=17\\ a^2 - 2b^2 &= -1 \end{cases}
Solving the system of equaitons gives
a = 2\sqrt{2}, b=3\sqrt{\dfrac{1}{2} }
Both results give the same root. Therefore,
The value of the square root of
\sqrt{17+12\sqrt{2}}=\sqrt{(3+2\sqrt{2})^2}=3+2\sqrt{2}
\sqrt{17-12\sqrt{2}}=\sqrt{(3-2\sqrt{2})^2}=3-2\sqrt{2}