Đáp án:

Bài 3 : 

`a) 14625 + 10657 + 25375 + 19343`

`= ( 14625 + 25375 ) + ( 10657 + 19343 )`

`= 40000 + 30000`

`= 700000`

`b, 13649 + 21532 + 36551 + 8468`

`= ( 13649 + 36551 ) + ( 21532 + 8468 )`

`= 50200 + 30000`

`= 80200`

`c, 25638 + 42735 + 7265 + 14762`

`= ( 25638 + 14762 ) + ( 42735 + 7265 )`

`= 40400 +  50000`

`= 90400`

`d, 542 + 348 + 125`

`= (542 + 348) + 125`

`= 890 + 125`

`= 1015`

`e, 129 + 658 + 871 + 342`

`= ( 129 + 871)  + (658 + 342)`

`= 1000 + 1000`

`= 2000`

`g, 46513 – (6513 – 297)`

`= 46513 - 6513 + 297`

`= 40000 + 297`

`= 40297`

`h, 38765 + 42576 – 8765 – 576`

`= ( 38765 - 8765 ) + ( 42576 - 567)`

`= 30000 + 42000`

`= 72000`

`i, 15237 – (1 0000 – 1763)`

`= 15237 - 10 000 + 1763`

`= 5237 + 1763`

`= 7000`

`k, 1345 + 678 + 655`

`= 1345 + 655 + 678`

`= 2000 + 678`

`= 2678`

Bài 4 :

`a, x + 6754= 12 087`

`-> x = 5333`

`b, 876 + x = 1099`

`-> x = 223`

`c, x - 457= 9876`

`-> x = 10333`

`d, 7621- x = 542`

`-> x = 7079`

`e, 3874 + x = 543 + 1975`

`-> 3874 + x = 2518`

`-> x = -1356`

`g, 8954 – x = 3690- 298`

`-> 8954 – x = 3392`

`-> x = 5562`

`h, x – 456= 234 + 4598`

`-> x - 456 = 4892`

`-> x = 5288`

`i, 8712 – x = 1234 – 432`

`-> 8712 - x = 802`

`-> x = 7910`

`k, 647 + x = 6532 x 4`

`-> 647 + x = 26128`

`-> x = 25481`

Bài 3: Tính nhanh:

a) 14625 + 10657 + 25375 + 19343

=(14625+25375)+(10657+19343)

=40 000               +  30 000

=      70 000

b) 13649 + 21532 + 36551 + 8468

=(13649+36551)+ (21532+8468)

=50200                 +   30000

= 80 200

c) 25638 + 42735 + 7265 + 14762.

=(25638+14762)+(42735+7265)

=40400+50000

=90400

d. 542 + 348 + 125

=(542+348)+125

=890+125

=1015

e.129 + 658 + 871 + 342 

=(129+871)+(658+342)

=1000+1000

=2000

g) 46513 – (6513 – 297)           (BỎ DẤU NGOẶC ĐỔI DẤU)

= 46513-6513+297

=(46513+297)-6513

=46800-6513

=40287

h) 38765 + 42576 – 8765 – 576

=(38765-8765)+(42576-576)

=30000+42000

=72000

k. 1345 + 678 + 655

=(1345+655)+678

=2000+678

=2678

i) 15237 – (1 0000 – 1763)

=15237-10000+1763

=(15237+1763)-10000

=17000-10000

=7000

Bài 4: Tìm x

X + 6754= 12 087

X                =12087-6754

X=5333

876 + X = 1099

X=1099-876

X=223

X- 457= 9876

X=9876+457

X=10333

7621- X = 542

X=7621-542

X=7079

3874 + X= 543 + 1975

3874 + X=2518

X=2518-3874

X=-1356

8954 – X = 3690- 298

8954 – X =3392

X=8954-3392

X=5562

X – 456= 234 + 4598

X – 456=4832

X=4832+456

X=5288

8712 – X = 1234 – 432

8712 – X =202

X=8510

647 + X = 6532 x4

647 + X =26128

X=25481