Bài 9: Tính nhanh tổng sau: a) 11+12+13++20.. b) 37.75 + 37.45 + 63.67 + 63.53 b) 11 + 13 + 15 +...+25. c) 35.34 + 35.86 + 65.75 + 65.45 c) 34 + 35 + 36 +...+43. d) 78.31+78.24+78.17+22.72. d) 50 + 51 + 52 +....+60.

Đáp án+Giải thích các bước giải:

 a)11+12+13+14+15+16+17+18+19+20

=(11+19)+(12+18)+(13+17)+(14+16)+(15+20)

=30+30+30+30+35

=155

b)37 . 75 + 37 . 45 + 63 . 67 + 63 . 53

= 37 . ( 75 + 45 ) + 63 . ( 67 + 53 )

= 37 . 120 + 63 . 120

= ( 37 + 63 ) . 120

= 100 . 120

= 12000

b)11+13+15+17+19+21+23+25

=(11+19)+(13+17)+(15+25)+(21+23)

=30+30+35+44

=139

c) 35.34 + 35.86 + 65.75 + 65.45

= 35. ( 34 + 86 ) + 65 . ( 75 + 25 )

= 35 . 120 + 65 . 100

= 4200 + 6500

= 10700

c)34+35+36+37+38+39+40+41+42+43

=(34+36)+(35+40)+(37+43)+(38+42)+(41+39)

=70+75+80+80+80

=385

d)78.31+78.24+78.17+22.72

=78.(31+24+17)+22.72

=78.72+22.72

=72.(78+22)

=72.100

=7200

d)50+51+52+53+54+55+56+57+58+59+60

=(50+55)+(51+59)+(52+58)+(53+57)+(54+56)+60

=105+110+110+110+110+60

=605