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(1+29)+(2+28)+(3+27)+(4+26)+(5+25)+(6+24)+(7+23)+(8+22)+(9+21)+(10+20)+(11+19)+(12+18)+(13+17)+(14+16)+15
=30+30+30+30+30+30+30+30+30+30+30+30+30+30+15
=30x14+15
=435
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a, 9 × 6 + 17 = 54 + 17 = 71
b, 9 × 3 × 4 = 27 × 4 = 108
c, 9 × 8 - 23 = 72 – 23 = 49
d, 9 × 9 ÷ 9 = 81 : 9 = 9
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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
=(1 + 19) + (2 +18) + (3 + 27) + (4 + 26) + (5 + 25) + (6 + 24) + (7 + 23) + (8 + 22) + (9 + 21) + (10 + 20) + (11 +19) + (12 + 18) + (13 + 17) + (14 + 16)
=20 + 20 + 30 + 30 + 30 + 30 + 30 + 30 + 30 + 30 + 30 + 30 + 30 + 15
=40 + 330 + 15
=381
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a. 1 x 9
= 9
b. 9 x 8 x 9
= (9 x 9) x 8
= 81 x 8
= 648
c. 10 x 8 x 10
= (10 x 10) x 8
= 100 x 8
= 800
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a/ 6 x 10 - 23 = 60 - 23 ( 0,5đ ) b/ 6 x 4 + 15 = 24 + 15 ( 0,5 đ )
= 37 ( 0,5đ ) = 39 ( 0,5đ )
23 x 10 +(1+9)
=230+10
=240
\(23\cdot10+(1+9)\)
\(=230+10=240\)