Question: simplification

I have this two expresions

 

e1 := (1/2)*P[psi]^2/(cos(`ϑ`)^2*Ix)

e2 := (1/2)*P[psi]^2/(cos(`ϑ`)^2*Ix)

 

simplify(e1-e2)=-(1/2)*(-P[psi]^2+P[psi]^2)/(cos(`ϑ`)^2*Ix)

 

but 

simplify(-P[psi]^2+P[psi]^2)

is zero 

 

why i dont obtain zero if i use simplify(e1-e2) ??

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