21 - ( - 23 ) + 25 - ( -27 ) + 29 - ( - 31 )
= 21 + 23 + 25 - ( - 27 ) + 29 - ( - 31 )
= 69 + 27 + 29 - ( - 31 )
= 125 + 31
= 156
21 - ( -23 ) + 25 - ( -27 ) + 29 - ( -31 )
= 21 + 23 + 25 + 27 +29 +31
= 156
=21+23+25-(-27)+29-(-31)
=69+27+29-(-31)
=125-31
=156
Ta có:
21 - ( - 23 ) + 25 - ( -27 ) + 29 - ( - 31 )
= 21 + 23 + 25 - ( - 27 ) + 29 - ( - 31 )
= 69 + 27 + 29 - ( - 31 )
= 125 + 31
= 156