56,78 x 13,45 - 13,45 x 47,78 + 13,45
= (56,78 - 47,78) x 13,45 + 13,45 x 1
= 9 x 13,45 + 13,45 x 1
= (9+1) x 13,45
= 10 x 13,45
= 134,5
1 + 2 + 3 + 4 + 5 +...... + 300
= (1 + 300) + (2 + 129) + (3 + 128) + (4 + 127) +.... (có 150 cặp)
= 301 x 150
= 45 150
Hok tốt !
# Moon
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