minus minus product why come plus

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i think everyone did algebra in the school. and we all know -.- = +. that mean minus into minus is plus. but do you know why minus into minus is plus. if you donot know then why you do not try to know it? and if know it then please share with me a have some confusion on it. i know one process and this is very complex. i solve it by complex variable. if you know any easy method then please give me this.