Question

tính nhanh:a)1+7+8+15+23+...+16

b)78.31+78.24+78.17+72.22

Answer 1

b) 78.31 + 78.24 + 78.17 + 72. 22

 = 78. ( 31 + 24 + 17 ) + 72 . 22

= 78 . 72 + 72 . 22

= 72 (78+22)

= 72 . 100 = 7200

Answer 2

b) 78 x 31 + 78 x 24 + 78 x 17 + 72 x 22 

= 78 x ( 31 + 24 + 17 ) + 72 x 22

= 78 x 72 + 72 x 22

= 72 x ( 78 + 22 ) 

= 72 x 100 

= 7200