Question

tính nhẩm theo các hằng đẳng thức các số sau

a. 19²;28²;81²;91²

b. 19.21;29.31;39.41

c.29²-8²;56²-46²;67²-56²

Answer 1

a) \(19^2=\left(20-1\right)^2=20^2-40+1=400-40+1=361\)
\(28^2=\left(30-2\right)^2=900-120+4=784\)
\(81^2=\left(80+1\right)^2=6400+160+1=6561\)
\(91^2=\left(90+1\right)^2=8100+180+1=8281\)
b) \(19.21=\left(20-1\right)\left(20+1\right)=20^2-1=400-1=399\)
\(29.31=\left(30-1\right)\left(30+1\right)=30^2-1=900-1=899\)
\(39.41=\left(40-1\right)\left(40+1\right)=40^2-1=1600-1=1599\)
c) \(29^2-28^2=\left(29+28\right)\left(29-28\right)=57\)
\(56^2-46^2=\left(56+46\right)\left(56-46\right)=102.10=1020\)
\(67^2-56^2=\left(67+56\right)\left(67-56\right)=123.11=1353\)