## Algebra and Trigonometry 10th Edition

Equation : $x^{4}-81=0$ $x^{4}-81=x^{4}-3^4=(x^2-3^2)(x^2+3^2)=(x^2-3^2)(x^2-(3i)^2)=0$ (as $i^2=-1$) Hence, $x^{4}-81=(x-3)(x+3)(x-3i(x+3i)=0$ Giving us 4 roots as 3, -3, 3i, -3i.