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Why does sqrt(4)-sqrt(16) not equal two? If sqrt(4) equals +-2 and sqrt(16) equals +-4, then could you not say -2+4 = 2? I love Neil deGrasse Tyson, but why is he always correcting the science in fictional movies? Why won't he let people have some fun? How can i find how much is my speed in the net work? HELP CHECK FOR EXTRANEOUS SOLUTIONS? Aluminum has a density of 171 lb/ft3. what would be its specify gravvity?