- Twelve people each shake hands with each other once. How many total handshakes are there?

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A relation ~ is defined on the set Z by the rule x~y if and only if x-y is a multiple of 4. Prove the relation ~ is reflexive and symmetric? A cable that is 39ft long goes from the ground to the top of bldg and forms an angle of 41.7 with the wall of the building. How many ft ? What would the world be like after an asteroid hits the earth? Is human extinction going to happen soon? Pls help me this solution...?