Toán Lớp 9: rút gọn biểu thức A=√(5+√21)+√(5-√21)-2√(4-√7)
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Toán Lớp 9: rút gọn biểu thức A=√(5+√21)+√(5-√21)-2√(4-√7)
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A = \sqrt {\left( {5 + \sqrt {21} } \right)} + \sqrt {\left( {5 – \sqrt {21} } \right)} – 2\sqrt {\left( {4 – \sqrt 7 } \right)} \\
A = \sqrt {4 + \sqrt 7 } + \sqrt {4 – \sqrt 7 } – 2\sqrt {\left( {4 – \sqrt 7 } \right)} \\
A = \sqrt {4 + \sqrt 7 } + \sqrt {4 – \sqrt 7 } – \sqrt {{{\left( {4 – \sqrt 7 } \right)}^2}} \\
A = \sqrt {4 + \sqrt 7 } + \sqrt {4 – \sqrt 7 } – \left| {4 – \sqrt 7 } \right|\\
A = \sqrt {4 + \sqrt 7 } + \sqrt {4 – \sqrt 7 } – 4 – \sqrt 7
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