Question 289931
{{{x^4-81x^2=0}}}
{{{x^2(x^2-81)=0}}}
Use the zero product rule,
{{{x^2=0}}}
{{{highlight(x=0)}}}
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.
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{{{x^2-81=0}}}
{{{x^2=81}}}
{{{highlight(x=-9)}}}, {{{highlight(x=9)}}}