Question 1015673
In your calculator, press .78.
Hit arc cos button.
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{{{sin(theta)=2/5}}}
So then,
{{{sin^2(theta)+cos^2(theta)=1}}}
{{{(2/5)^2+cos^2(theta)=1}}}
{{{4/25+cos^2(theta)=1}}}
{{{cos^2(theta)=21/25}}}
{{{cos(theta)=0 +- sqrt(21)/5}}}

So then,
{{{tan(theta)=(2/5)/(0 +- sqrt(21)/5)}}}
{{{tan(theta)=0 +- 2/sqrt(21)}}}
{{{tan(theta)=0 +- (2/21)sqrt(21)}}}