21+23+25+……+143
=1+3+5+7+9+11+13+15+17+19+21+23+25+....+143-(1+3+5+7+...+19)
=[(1+143)÷2]²-[(1+19)÷2]²
=77²-10²
=(77-10)(77+10)
=67×87
=5829
21+23+25+……+143
=1+3+5+7+9+11+13+15+17+19+21+23+25+....+143-(1+3+5+7+...+19)
=[(1+143)÷2]²-[(1+19)÷2]²
=77²-10²
=(77-10)(77+10)
=67×87
=5829