1/1991+ 1992*1990/1991 - 1992
without doing 4digit by 4digit multiplication
SHOW ME HOW the awnser -1
+118658 Do you seriously expect anyone to look at your questions when you ask in such a derogatory way?
+9665 Good question though.
\(\text{Let }x = 1991\\ \dfrac{1}{1991} + \dfrac{1992\times 1990}{1991} - 1992\\ = \dfrac{1}{x} + \dfrac{(x+1)(x-1)}{x}-(x+1)\\ =\dfrac{1}{x} + \dfrac{x^2 - 1}{x} - (x+1)\\ = \dfrac{x^2 +1 - 1}{x }-(x+1)\\ = \dfrac{x^2}{x} -(x+1)\\ = x - x - 1\\ = -1\)
Substitution is always brilliant :)
