Tính nhanh: a) 92.17 + 83.92; b) 108.12 + 25.92 + 13. 108
Giải thích
a) 92.17 + 83.92
= 92. ( 17 + 83)
= 92 . 100
= 9200
b) 108.12 + 25.92 + 13. 108
= 108.(12 + 13) + 25.92
= 108. 25 + 25.92
= 25. (108 + 92)
= 25. 200
=5000
c) 1 + 3 + 5 +... + 39
= 20.(39+1):2
= 400
d) 2 + 7 + 12 + 17 + …+ 62.
= (2 + 62) + (7+57) + ....(27+ 37)+32
= 64 + 64 + ...+64+32
= 64.6+32
= 416