98 x 8 + 98 x 13 - 98

= 98 x ( 13 + 8 - 1 )

= 98 x 20

= 1960

125 x 63 x 8

= 63 x ( 125 x 8 )

= 63 x 1000

= 63000

a, 98x8+98x13-98

=98x8+98x13-98x1

=98x[8+13-1]

=98x20

=1960

b,125x63x8

=[125x8]x63

=1000x63

=63000

16 x 98 + 16 x 2

= 16 x ( 98 + 2 )

= 16 x 100

= 1600

16 x 98 + 16 x 2

= 16 x ( 98 + 2 ) 

= 16 x 100 = 1600

a: =100x26=2600

b: =78x10=780

c: =67x100=6700

d: =1000x575=575000

\(4\times26\times25=\left(4\times25\right)\times26=100\times26=2600\\ 63\times78-53\times78=\left(63-53\right)\times78=10\times78=780\\ 67\times92+67\times8=67\times\left(92+8\right)=67\times100=6700\\ 8\times23\times25\times125=\left(8\times125\right)\times\left(23\times25\right)=1000\times575=57500\)

( 1 + 2 + 3 + 3 + ... + 97 + 98 ) x ( 45 x 3 - 45 x 2 - 45 )

= ( 1 + 2 + 3 + 3 + ... + 97 + 98 ) x 0 (mik làm tắt phần này bạn tự làm)

= 0

(1 + 2 + 3 + 3 + ... + 97 + 98) x (45 x 3 - 45 x 2 - 45)

= (1 + 2 + 3 + 3 + ... + 97 + 98) x [45 x (3 - 2 - 1)]

= (1 + 2 + 3 + 3 + ... + 97 + 98) x (45 x 0)

= (1 + 2 + 3 + 3 + ... + 97 + 98) x 0

= 0.

Bài 8: Tính bằng cách thuận tiện nhất:
a, 6 x 31 x 5 =(6x5)x31
                    =30x31
                    =930

b, 8 x 63 x 5 =(8x5)x63
                     =40x63
                     =2520

a.

6 x 31 x 5

= (6 x 5) x 31

= 30 x 31

= 930

b.

8 x 63 x 5

= (8 x 5) x 63

= 40 x 63

= 2520

13/19 x 23/11 - 13/19 x 8/11 - 13/19 x 4/11

= 13/19 x (23/11 - 8/11 - 4/11)

= 13/19 x 1

= 13/19

\(\dfrac{13}{19}\times\dfrac{23}{11}-\dfrac{13}{19}\times\dfrac{8}{11}-\dfrac{13}{19}\times\dfrac{4}{11}\\ =\dfrac{13}{19}\times\left(\dfrac{23}{11}-\dfrac{8}{11}-\dfrac{4}{11}\right)\\ =\dfrac{13}{19}\times1=\dfrac{13}{19}\)

Hướng dẫn giải:

a) 4 x 78 x 5 = (4 x 5) x 78 = 20 x 78 = 1560

b) 2 x 99 x 5 = (2 x 5) x 99 = 10 x 99 = 990

c) 6 x 31 x 5 = (6 x 5) x 31 = 30 x 31 = 930

d) 8 x 63 x 5 = (8 x 5) x 63 = 40 x 63 = 2520