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[tex]\begin{gathered} \sqrt[]{-49} \\ \text{Use this property:} \\ \sqrt[]{a\cdot b}=\sqrt[]{a}\cdot\sqrt[]{b} \\ So\colon \\ \sqrt[]{-49}=\sqrt[]{49\cdot(-1)}=\sqrt[]{49}\cdot\sqrt[]{-1} \\ \text{Where:} \\ \sqrt[]{49}=7 \\ \sqrt[]{-1}=i \\ so\colon \\ \sqrt[]{49}\cdot\sqrt[]{-1}=7i \end{gathered}[/tex]