Question

Tính bằng cách thuận tiện:

a)11 - 12 + 13 - 14 + 15 - 16 + 17 - 18 + 19 - 20 + 21

b)158 + 289 + 355 + 257 + 242 + 445 + 833 + 251 - 100

c)1000 - 900 + 800 - 700 + 600 - 500 + 400 - 300 + 200 - 100

Answer 1

a) 11 - 12 + 13 - 14 + 15 - 16 + 17 - 18 + 19 - 20 + 21

= 11 + (13 - 12) + (15 - 14) + (17 - 16) + (19 - 18) + (21 - 20)

= 11 + 1 + 1 + 1 + 1 + 1 

= 16

b) 158 + 289 + 355 + 257 + 242 + 445 + 833 + 251 - 100

= (158 + 242) + (355 + 445) + (289 + 251) + (257 + 833) - 100

= 400 + 800 + 540 + 1080 - 100

= 1200 + 1620 - 100

= 2820 - 100

= 2720

c) 1000 - 900 + 800 - 700 + 600 - 500 + 400 - 300 + 200 - 100

= (1000 - 900) + (800 - 700) + (600 - 500) + (400 - 300) + (200 - 100)

= 100 + 100 + 100 + 100 + 100

= 500