Principal square root

Definition

The positive square root represented by the symbol $\sqrt{\phantom{x}}$ ; for example, $\sqrt{25}=5$ .

Specification N7 · Paper 1, Paper 2, Paper 3

Specification N6 · Paper 1, Paper 2, Paper 3

The non-negative square root represented by the symbol $\sqrt{\phantom{x}}$ ; for example, the principal square root of is .

1
Do not write $\sqrt{81}=\pm9$ . The radical symbol gives the principal square root, so ; the equation has solutions .

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25=

5

Used by AQA and Edexcel

x

81

9

=

±9

\sqrt{81}=9

x^2=81

x=\pm9