Question

\(\dfrac{1989\times1990+3978}{1992\times1991-3984}\)

Answer 1

\(\text{Ta có : }\dfrac{1989\cdot1990+3978}{1992\cdot1991-3984}=\dfrac{1989\cdot1990+1989\cdot2}{1992\cdot1991-1992\cdot2}\\ =\dfrac{1989\left(1990+2\right)}{1992\left(1991-2\right)}\\ =\dfrac{1989\cdot1992}{1992\cdot1989}\\ =1\)

Answer 2

Đặt:

\(A=\dfrac{1989.1990+3978}{1992.1991-3984}\)

\(A=\dfrac{1989.1990+1989.2}{1992.1991-3984}\)

\(A=\dfrac{1989\left(1990+2\right)}{1992.1991-3984}\)

\(A=\dfrac{1989.1992}{1992.1991-3984}\)

\(A=\dfrac{1989.1992}{1992\left(1989+2\right)-3984}\)

\(A=\dfrac{1989.1992}{1992.1989+1992.2-3984}\)

\(A=\dfrac{1989.1992}{1992.1989+3984-3984}\)

\(A=\dfrac{1989.1992}{1992.1989}=1\)