Giải Bài 5 trang 22

Đề bài

Tính nhanh:

a) $38.42$                               b) ${102}^2$                          c) ${198}^2$              d) ${75}^2-{25}^2$

Lời giải chi tiết

a) $38.42=\left(40-2\right).\left(40+2\right)={40}^2-2^2=1600-4=1596$

b) ${102}^2={\left(100+2\right)}^2={100}^2+\mathrm{2.100.2}+2^2=10000+400+4=10404$

c) ${198}^2={\left(200-2\right)}^2={200}^2-\mathrm{2.200.2}+2^2=40000-800+4=39204$

d) ${75}^2-{25}^2=\left(75+25\right)\left(75-25\right)=100.50=5000$