Question

simplificando a expressão

Answer 1

$A=(\sqrt{5+\sqrt{21}})\cdot(\sqrt{5-\sqrt{21}})$

Elevando os dois lados ao quadrado:

$A^2=(\sqrt{5+\sqrt{21}})^2\cdot(\sqrt{5-\sqrt{21}})^2$

$A^2=(5+\sqrt{21})\cdot(5-\sqrt{21})$

$A^2=5^2-(\sqrt{21})^2$

$A^2=25-21$

$A^2=4$

$A=\sqrt{4}$

$A=2$

$\boxed{\text{Letra C}}$

Answer 2

  

$A= \sqrt{(5+ \sqrt{21})(5- \sqrt{21} ) } \\ \\ A= \sqrt{5^2- \sqrt{21^2} } \\ \\ A= \sqrt{25-21} \\ \\ A= \sqrt{4} \\ \\ A=2 \\ \\ Letra~~C$