Question

Tính nhẩm :

a) 49 . 51

b) 55² - 45²

P/s : áp dụng hàng đẳng thức.

Answer 1

\(a,=\left(50-1\right)\left(50+1\right)\)

\(=50^2-1\)

\(=2499\)

\(b,55^2-45^2\)

\(=\left(55-45\right)\left(55+45\right)\)

\(=10.100\)

\(=1000\)

Answer 2

\(\text{a) }49.51=\left(50-1\right)\left(50+1\right)=50^2-1^2=2500-1=2499\)

\(\text{b) }55^2-45^2=\left(55+45\right)\left(55-45\right)=100.10=1000\)

Answer 3

\(49\cdot51\)

\(=\left(50-1\right)\cdot\left(50+1\right)\)

\(=50^2-1\)

\(=2499\)

Answer 4

\(55^2-45^2\)

\(=\left(55-45\right)\cdot\left(55+45\right)\)

\(=10\cdot100\)

\(=1000\)