537-(343-263)-57 1+3+5+…+35+37+39.
问题描述:
537-(343-263)-57
1+3+5+…+35+37+39.
答
(1)537-(343-263)-57
=537-343+263-57,
=537+263-(343+57),
=800-400,
=400;
(2)1+3+5+…+35+37+39
=(1+39)×[(39-1)÷2+1]÷2,
=40×[40÷2+1]÷2,
=40×[19+1]÷2,
=40×20÷2,
=400.