Câu 1:
\(a,B=\left[\sqrt{a}-\dfrac{\left(\sqrt{a}+1\right)\left(a-\sqrt{a}+1\right)}{\sqrt{a}+1}\right]\left[\sqrt{a}+\dfrac{\left(\sqrt{a}-1\right)\left(a+\sqrt{a}+1\right)}{\sqrt{a}-1}\right]\\ B=\left(-a+2\sqrt{a}-1\right)\left(a+2\sqrt{a}+1\right)\\ B=-\left(\sqrt{a}-1\right)^2\left(\sqrt{a}+1\right)^2=-\left(a-1\right)^2\\ b,B=25\Leftrightarrow-\left(a-1\right)^2=25\left(\text{vô lí}\right)\\ \Leftrightarrow a\in\varnothing\)
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