how come when i type in 1/tan 90 i get undefined on my ti-84

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Tony -The limit of the Tan(x) function is equal to zero as x approaches π/2 from either the left or right.HOWEVER, the function Tan (π/2) is undefined at the single point π/2 because the function goes to infinity which is undefined as far as functions go. Dividing by an undefined function is also undefined.Now, if instead the problem stated:1 ÷ (sin 90 ÷ cos 90)This would simply equal cos 90 ÷ sin 90 which does equal zero.Hope that helps