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Tính nhanh mỗi biểu thức sau:
a, 0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20
= (0 + 20) + (1 + 19) + (2 + 18) + (3 + 17) + (4 + 16) + (5 + 15) + (6 + 14) + (7 + 13) + (8 + 12) + (9 + 11) + 10
= 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 20 + 10
= 20 x 10 + 10
= 200 + 10
= 210
b, 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x (4 x 9 - 36)
= 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x (36 - 36)
= 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 0
= A x 0
= 0
c, (81 - 7 x 9 - 18) : (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)
= (81 - 63 - 18) : (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)
= (18 - 18) : (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)
= 0 :(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)
= 0 : A
= 0
d, (6 x 5 + 7 - 37) x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)
= (30 + 7 - 37) x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)
= (37 - 37) x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)
= 0 x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10)
= 0 x A
= 0
e, (11 x 9 - 100 + 1) : (1 x 2 x 3 x 4 x ... x 10)
= (99 - 100 + 1) : (1 x 2 x 3 x 4 x ... x 10)
= (99 + 1 - 100) : (1 x 2 x 3 x 4 x ... x 10)
= (100 - 100) : (1 x 2 x 3 x 4 x ... x 10)
= 0 : (1 x 2 x 3 x 4 x ... x 10)
= 0 : A
= 0
g, (m : 1 - m x 1) : (m x 2008 + m x 2008)
= (m - m) : (m x 2008 + m x 2008)
= 0 : (m x 2008 + m x 2008)
= 0 : A
= 0
h, (2 + 4 + 6 + 8 + m x n) x (324 x 3 - 972)
= (2 + 4 + 6 + 8 + m x n) x (972 - 972)
= (2 + 4 + 6 + 8 + m x n) x 0
= A x 0
= 0
l, (1 + 2 + 3 + ... + 99) x (13 x 15 - 12 x 15 - 15)
= (1 + 2 + 3 + ... + 99) x (15 x (13 - 12 - 1))
= (1 + 2 + 3 + ... + 99) x (15 x 0)
= (1 + 2 + 3 + ... + 99) x 0
= A x 0
= 0
i, (0 x 1 x 2 x...x 99 x 100) : (2 + 4 + 6 +...+ 98)
= 0 x : (2 + 4 + 6 +...+ 98)
= 0 x A
= 0
k, (0 + 1 + 2 +...+ 97 + 99) x (45 x 3 - 45 x 2 - 45)
= (0 + 1 + 2 +...+ 97 + 99) x (45 x (3 - 2 - 4))
= (0 + 1 + 2 +...+ 97 + 99) x (45 x 0)
= (0 + 1 + 2 +...+ 97 + 99) x 0
= A x 0
= 0
Bài 1:
Chiều rộng hcn đó là:
\(\dfrac{4}{3}\times\dfrac{1}{2}=\dfrac{2}{3}\) ( m )
Chu vi hình chữ nhật đó là:
\(\left(\dfrac{4}{3}+\dfrac{2}{3}\right)\times2=4\left(m\right)\)
Đ/s: 4 m
Bài 2:
\(\dfrac{2}{5}+\dfrac{1}{3}:\dfrac{2}{3}=\dfrac{2}{5}+\dfrac{1}{3}\times\dfrac{3}{2}=\dfrac{2}{5}+\dfrac{1}{2}=\dfrac{4}{10}+\dfrac{5}{10}=\dfrac{9}{10}\)
Bài 3;
\(\Rightarrow\dfrac{3}{4}+x\times\dfrac{5}{2}=\dfrac{37}{16}\\ \Rightarrow x\times\dfrac{5}{2}=\dfrac{37}{16}-\dfrac{3}{4}=\dfrac{25}{16}\\ \Rightarrow x=\dfrac{25}{16}\times\dfrac{2}{5}=\dfrac{5}{8}\)
Bài 4:
\(\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}=\dfrac{15}{16}\)
Bài 5:
\(\Rightarrow x\times\dfrac{2}{5}=\dfrac{4}{7}\\ \Rightarrow x=\dfrac{4}{7}\times\dfrac{5}{2}=\dfrac{10}{7}\)
bài 1
a)1 ngày 6 giờ= 30 giờ
1/6 ngày= 4 giờ
b) 25dm250cm2=150cm2
15km2342m2=15000342m2
bài 2
a) (36+54)x7+7x9+7=90x7+63+7=630+70=500
b)3/4 x 3/9 + 3/4 x 2/3=3/4x1/3+3/4x2/3 =3/4x(1/3+2/3)=3/4x1=3/4
c) 85x72+84x27+84=85x72+84x(27+1)=6120+84x28=6120+2352=8472
Bài 1:
a, 1 ngày 6 giờ = 30 giờ.
1/6 ngày = 4 giờ.
b, 25 dm2 50m cm2 = 2550 cm2
15 km2 342 m2 = 15 000 342 m2.
Bài 2: Chịu.
\(\dfrac{2}{3}+\dfrac{18}{15}\times\dfrac{5}{16}\)
\(=\dfrac{2}{3}+\dfrac{18\times5}{15\times16}\)
\(=\dfrac{2}{3}+\dfrac{9\times1}{3\times8}\)
\(=\dfrac{2}{3}+\dfrac{9}{24}\)
\(=\dfrac{16}{24}+\dfrac{9}{24}\)
\(=\dfrac{25}{24}\)
_______________________
\(\dfrac{4}{7}-\dfrac{3}{7}:\dfrac{4}{3}\)
\(=\dfrac{4}{7}-\dfrac{9}{28}\)
\(=\dfrac{16}{28}-\dfrac{9}{28}\)
\(=\dfrac{7}{28}=\dfrac{1}{4}\)
Chúc bạn học tốt
2/3 + 3/4=17/12
9/4 + 3/5=57/20
5/24 + 1/4=11/24
3/15 - 5/35=2/35 18/27 - 2/6=1/3 37/12 - 3=1/12
11/9 x 3/22=1/6 7/13 x 13/7=1 4 x6/7=24/7
2/5 : 3/10=4/3 3/8 : 9/4=1/6 8/21 : 4/7=2/3
2 x m x 3 = 456
m x 2 x 3 = 456
m x 6 = 456
m = 456 : 6
m = 76
m : 3 - 207 =102
m : 3 = 102 + 207
m : 3 = 309
m = 309 x 3
m = 927
235 : m x 3 = 15
235 : m = 15 : 3
235 : m = 5
m = 235 : 5
m = 47
m : 4 : 2 =18
m : 4 = 18 x 2
m : 4 = 36
m = 36 x 4
m = 144
2 x m x 3 = 456
2 x m = 456 : 3
2 x m = 152
m = 152 : 2
m = 76
m : 3 - 207 = 102
m : 3 = 102 + 207
m : 3 = 309
m = 309 x 3
m = 927
235 : m x 3 = 15
235 : m = 15 : 3
235 : m = 5
m = 235 : 5
m = 47
m : 4 : 2 = 18
m : 4 = 18 x 2
m : 4 = 36
m = 36 x 4
m = 144
~ HT ~