30-28+26-24+22-20+18-16+14-12+10-8+4-2
=(30-28)+(26-24)+(22-20)+(18-16)+(14-12)+(10-8)+(4-2)
=2+2+2+2+2+2+2
=\(2\times7\)
=14
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tinh nhanh
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2+4+6+8+10+12+14+16+18+20+22+24+26+28+30+32+34+36+38+40+42+44+46+48=??
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hãy tính nhanh:
A.1+2+3+4+5+6+7+8+9+10+11+12+13-14=?
B.30-29+28-27+26-25+24-23+22-21...+12-11+10-9+8-7+6-5+4-3+2-1=?
28+27+26+25+24+23+22+21+20+19+18+17+16+15+14+13+12+11+10+9+8+7+6+5+4+3+2+1+=....................................................?
1.Tính nhanh:
a) 9-10+11-12+13-14+15
b)22+24+26-28+30
1/Tính nhanh:
32-30+28-26+24-22+20-18
20-18+16-14+12-10+8-6+4-2 tính nhanh
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30+31+32+33+34+35+36+37+38+39+40+41+42+43+44+45+46+47+48+49+50+51+52+53+54+56+57+58+59+60+61+62+63+64+65+66+67+68+69+70=?