Question

tính nhẩm theo hằng đẳng thức các số sau

a.\(19^2;28^2;81^2;91^2\)

b.19.21;29.31;39.41

c.29\(^2-8^2;56^2-46^2;67^2-56^2\)

Answer 1

a)

\(19^2=\left(20-1\right)^2=20^2-2.20.1+1^2=400-40+1=361\)

\(28^2=\left(30-2\right)^2=30^2-2.30.2+2^2=900-120+4=784\)

\(81^2=\left(80+1\right)^2=80^2+2.80.1+1^2=6400+160+1=6561\)

\(91^2=\left(90+1\right)^2=90^2+2.90.1+1^2=8100+180+1=8281\)

b)

\(19.21=\left(20-1\right)\left(20+1\right)=20^2-1^2=400-1=399\)

\(29.31=\left(30-1\right)\left(30+1\right)=30^2-1^2=900-1=899\)

\(39.41=\left(40-1\right)\left(40+1\right)=40^2-1^2=1600-1=1599\)

P/s: Lần sau cậu nên chia nhỏ ra đăng nhé!