Let S={1,2,3}. Then ∅⊆ S. Why is this true?

∅⊆{∅}. Why is this true?

The set {x∈ ℚ : 2x2 - 7x- 4 =0}. How come this one has 2 elements instead of 1?

Thanks.

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- September 20th 2011, 12:30 AMnaturebuilderjust a few simple questions on set
Let S={1,2,3}. Then ∅⊆ S. Why is this true?

∅⊆{∅}. Why is this true?

The set {x∈ ℚ : 2x2 - 7x- 4 =0}. How come this one has 2 elements instead of 1?

Thanks. - September 20th 2011, 02:42 AMPlatoRe: just a few simple questions on set