(1) 499^2=(500-1)^2=500^2+1^2-2*500=249001
(2) 998^2=(1000-2)^2=1000^2+2^2-2*1000*2=996004
(3) 53^2=(50+3)^2=50^2+3^2+2*50*3=2809
(4) 88^2=(90-2)^2=90^2+2^2-2*90*2=7744
^2指平方
(1) 499^2=(500-1)^2=500^2+1^2-2*500=249001
(2) 998^2=(1000-2)^2=1000^2+2^2-2*1000*2=996004
(3) 53^2=(50+3)^2=50^2+3^2+2*50*3=2809
(4) 88^2=(90-2)^2=90^2+2^2-2*90*2=7744
^2指平方