Question

399*45*55*399 1994*1996-1991*1995

Answer 1

399*45+55*399=399*100=39900

1994*1996-1991*1995

=1995^2-1-1991*1995

=1995*4-1

=7979

Answer 2

Sửa đề nhé: \(\dfrac{399\times45+55\times399}{1995\times1996-1991\times1995}\)

\(399\times45+55\times399=399\times\left(45+55\right)=399\times100\)                 (1)

\(1995\times1996-1991\times1995=1995\times\left(1996-1991\right)\) 

\(=1995\times5=\left(2000-5\right)=\left(400\times5\right)-1\times5=5\times\left(400-1\right)=5\times399\)   (2)

Từ (1) và (2) \(\Rightarrow\dfrac{399\times100}{399\times5}=\dfrac{100}{5}=20\)