교묘 한 계산.19+199+1999+1999+1991+2+3+4+...+97+98+99+100.

교묘 한 계산.19+199+1999+1999+1991+2+3+4+...+97+98+99+100.

19+199+1999+19999+199,=19+1+199+1+1999+1+19999+1+199+1-5,=20+200+2000+20000+200-5,=22420-5,=22415;1+2+3+4+…+97+98+99+100,=(1+100)×100÷2,=101×100÷2,=5050.