Question:
Find the value
$21 x^{2}-2 x+1 / 21$
Solution:
$=(\sqrt{21 x})^{2}-2 \sqrt{21} x \times \frac{1}{\sqrt{21}}+\left(\frac{1}{\sqrt{21}}\right)^{2}$
Using the identity $(x-y)^{2}=x^{2}+y^{2}-2 x y$
$=\left(\sqrt{21} x-\frac{1}{\sqrt{21}}\right)^{2}$
$\therefore 21 \mathrm{x}^{2}-2 \mathrm{x}+\frac{1}{21}=\left(\sqrt{21} \mathrm{x}-\frac{1}{\sqrt{21}}\right)^{2}$
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