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1、MATHEEEMAGICSWorkbookn3tfl3x7d6@yahoo.comCHAPTER1MENTALADDITIONIrememberthedayinthirdgradewhenIdiscoveredthatitwaseasiertoaddandsubtractfromlefttorightthanfromrighttoleft,whichwasthewaywehadallbeentaught.SuddenlyIwasabletoblurtouttheanswerstomathproblemsinclasswellbeforemyclassmatesputdowntheir
2、pencils.AndIdidn'tevenneedapencil!ThemethodwassosimplethatIperformedmostcalculationsinmyhead.Lookingback,IadmitIdidsoasmuchtcshowoffasforanymathematicalreason.Mostkidsoutgrowsuchbehavior.Thosewhodon'tprobablybecomeeitherteachersormagicians.Inthischapteryouwilllearntheleft-to-rightmethodofdoingm
3、entaladditionfornumbersthatrangeinsizefromtwotofourdigits.Thesementalskillsarenotonlyimportantfordoingthetricksinthisbookbutarealsoindispensableinschooloratwork,oranytimeyouusenumbers.Soonyouwillbeabletoretireyourcalculatorandusethefullcapacityofyourmindasyouadd,subtract,multiply,anddivide2-dig
4、it,3-digit,andeven4-digitnumbers.LEFT-TO-RIGHTADDITIONTherearemanygoodreasonswhyaddinglefttorightisasuperiormethodformentalcalculation.Foronething,youdonothavetoreversethenumbers(asyoudowhenaddingrighttoleft).Andifyouwanttoestimateyouranswer,thenaddingonlytheleadingdigitswillgetyouprettyclose.I
5、fyouareusedtoworkingfromrighttoleftonpaper,itmayseemunnaturaltoaddandmultiplyfromlefttoright.Butwithpracticeyouwillfindthatitisthemostnaturalandefficientwaytodomentalcalculations.Withthefirstsetofproblems—2digitaddition—thelefttorightmethodmaynotseemsoadvantageousn3tfl3x7d6.Butbepatient.Ifyoust
6、ickwithme,youwillseethattheonlyeasywaytosolve3-digitandlargeradditionproblems,allsubtractionproblems,andmostdefinitelyallmultiplicationanddivisionproblems,isfromlefttoright.Thesooneryougetaccustomedtocomputingthisway,thebetter.2-DIGITADDITIONOurassumptioninthischapteristhatyouknowhowtoaddandsub
7、tract1-digitnumbers.Wewillbeginwith2-digitaddition,somethingIsuspectyoucanalreadydofairlywellinyourhead.Thefollowingexercisesaregoodpractice,however,becauseyouwillusethe2-digitadditionskillsyoupolishhereforlargeradditionproblems,a
