Lời giải:
\(P=\sqrt{14+\sqrt{40}+\sqrt{56}+\sqrt{140}}=\sqrt{14+2\sqrt{10}+2\sqrt{14}+2\sqrt{35}}\)
\(=\sqrt{(7+2\sqrt{7.5}+5)+2(\sqrt{10}+\sqrt{14})+2}\)
\(=\sqrt{(\sqrt{7}+\sqrt{5})^2+2\sqrt{2}(\sqrt{5}+\sqrt{7})+(\sqrt{2})^2}\)
\(=\sqrt{(\sqrt{5}+\sqrt{7}+\sqrt{2})^2}=\sqrt{5}+\sqrt{7}+\sqrt{2}\)