Giải:
a) \(\left(31,8\right)^2-2.31,8.21,8+\left(21,8\right)^2\)
\(=\left(31,8-21,8\right)^2\)
\(=10^2=100\)
Vậy ...
b) \(\left(7,5\right)^2+2.7,5.2,5+\left(2,5\right)^2\)
\(=\left(7,5+2,5\right)^2\)
\(=10^2=100\)
Vậy ...
HĐT: \(\left(A\pm B\right)^2=A^2\pm2.A.B+B^2\)
a, (31,8)2 - 2.31,8.21,8+(21,8)2
=(31,8+21,8)2 =102 =100
b,(7,5)2 + 2.7,5.2,5 +(2,5)2
=(7,5 +2,5)2 =102 =100