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First, transform `sec^2(x)` into `1+tan^(x)` (this is obvious):
`1+tan^2(x)-6tan(x)=-4,` or `tan^2(x)-6tan(x)+5=0.`
This is a quadratic equation for tan(x), its roots are 1 and 5.
So we have tan(x)=1 or tan(x)=5.
On `(0, 2pi)` there are four roots: `pi/4,` `(5pi)/4,` `arctan(5)` and `pi+arctan(5).`
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