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25 + 25 + 25 + 25 + 25 + 25 +25 + 25=25x8=200
45 + 45 + 45 + 45 + 15 + 15 + 15 + 15=(45x4)+(15x4)=180+60=240
48 x 25 + 53 x 25 - 25
= 48 x 25 + 53 x 25 - 25 x 1
= 25 x ( 48 + 53 - 1 )
= 25 x ( 101 - 1 )
= 25 x 100
= 2500
= 25 x 25 + 25 x 75 = 25x (25 + 75) = 25 x 100 = 2500
a, 75 x 75 + 75 x 24 + 75
= 75 x ( 75 + 24 + 1 )
= 75 x 100
= 7500
b, 25 × 25 + 25 × 25 + 25 × 25 + 25 × 24 + 25
= 25 x ( 25 + 25 + 25 + 24 + 1 )
= 25 x 100
= 2500
c, 125 x 4 x 25 x 8
= ( 125 x 8 ) x ( 4 x 25 )
= 1000 x 100
= 100000
a) = 75 x (75+24+1)
=75 x 100
=7500
b) =25 x (25+25+25+24+25)
= 25 x 124
= 3100
c) = (125 x 8) x (25 x 4)
=1000 x 100
=100000
1 : Tính nhanh:
a) 2376 + 3425 - 376 - 425 = ( 2376 - 376 ) + ( 3425 - 425 )
= 2000 + 3000
= 5000
b)4638 - 2437 + 5362 - 7563 = 2210 + ( - 2210 )
= 0
c) 45 + 45 + 45 + 45 + 15 + 15 + 15 + 15
= 45 x 4 + 15 x 4
= ( 45 + 15 ) x 4
= 60 x 4
= 240
d) 4 x 125 x 25 x 8 = ( 4 x 25 ) x ( 125 x 8 )
= 100 x 1000
= 100000
e) 2 x 8 x 50 x 25 x 125 = ( 2 x 50 ) x ( 8 x 125 ) x 25
= 100 x 1000 x 25
= 100000 x 25
= 2500000
2 : Tính nhanh bằng 2 cách:
C1 : 125 + 125 + 125 + 125 - 25 - 25 - 25 - 25
= ( 125 - 25 ) + ( 125 - 25) + ( 125 - 25 ) + ( 125 - 25 )
=100 + 100 + 100 + 100
= 100 x 4
= 400
C2 : 125 + 125 + 125 + 125 - 25 - 25 - 25 - 25
= 125 x 4 - 25 x 4
= ( 125 - 25 ) x 4
= 100 x 4
= 400
\(\text{Ta có : }\)\(\frac{25}{4.9}+\frac{25}{9.14}+\frac{25}{14.19}+......+\frac{25}{2004.2009}\)
\(=5.\left(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+......+\frac{5}{2004.2009}\right)\)
\(=5\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+......+\frac{1}{2004}-\frac{1}{2009}\right)\)
\(=5\left(\frac{1}{4}-\frac{1}{2009}\right)\)
\(=5.\frac{2005}{8036}=\frac{10025}{8036}\)
\(...=25x10=250\)
25 + 25 + 25 + 25 + 25 + 25 + 25 + 25 + 25 + 25
= 25 x 10
= 250