3,1 . 28 - 72 . 3,1-21 + (-3100)
Tinh nhanh
3,1 . 28 - 72 . 3,1 - 21 + (-3100)
Đưa các số thập phân vô hạn tuần hoàn sau về phân số
a. 3,1(21)
b. -1,(28)
Ta có:
\(3,1\left(21\right)=3,1+0,0\left(21\right)=\frac{31}{10}+0,0\left(01\right).21=\frac{31}{10}+\frac{0,\left(01\right)}{10}.21=\frac{31}{10}+\frac{\frac{1}{99}}{10}.21\)
\(=\frac{31}{10}+\frac{21}{9900}=\frac{3069}{9900}+\frac{21}{9900}=\frac{3090}{9900}=\frac{103}{330}\)
0,2 x 17 x 7 + 0,14 x 520 + 3,1 x 14
tinh nhanh
loi giai day du
=(0,2 x 7) x17 + 0,14 x10x52+3,1 x 10x1,4
=1,4x17+1,4x52+31x1,4
=1,4x(17+52+31)
=1,4x100=140
Tính giá trị một cách hợp lý
A = 4,8 . ( 3,1 - 1,5 ) + 1.5 . ( 4,8 - 3,1 )
B = \(\dfrac{2^{19}.27^3+15.4^9.9^4}{6^9.2^{10}+12^{10}}\)
C = -1 -\(\dfrac{1}{3}-\dfrac{1}{6}-\dfrac{1}{10}-\dfrac{1}{15}-\dfrac{1}{21}-\dfrac{1}{28}-\dfrac{1}{36}-\dfrac{1}{45}\)
\(A=4,8.\left(3,1-1,5\right)+1,5.\left(4,8-3,1\right)\)
\(A=4,8.3,1-4,8.1,5+1,5.4,8-1,5.3,1\)
\(A=3,1.\left(4,8-1,5\right)-4,8\left(1,5+1,5\right)\)
\(A=3,1.3,3-4,8.3\)
\(A=10,23-14,4=-4,17\)
\(B=\dfrac{2^{19}.27^3+15.4^9.9^4}{6^9.2^{10}+12^{10}}=\dfrac{2^{19}.\left(3^3\right)^3+3.5.\left(2^2\right)^9.\left(3^2\right)^4}{\left(2.3\right)^9.2^{10}+\left(3.2^2\right)^{10}}=\dfrac{2^{19}.3^9+3.5.2^{18}.3^8}{2.3^9.2^{10}+3^{10}.\left(2^2\right)^{10}}=\dfrac{2^{19}.3^9+3^9.2^{18}.5}{2^{11}.3^9+3^{10}.2^{20}}=\dfrac{2^{18}.3^9\left(2+5\right)}{2^{11}.3^9\left(1+3.2^9\right)}=\dfrac{2^7.7}{1+3.2^9}\)
\(C=-1-\dfrac{1}{3}-\dfrac{1}{6}-\dfrac{1}{10}-\dfrac{1}{15}-\dfrac{1}{21}-\dfrac{1}{28}-\dfrac{1}{36}-\dfrac{1}{45}\)
\(\Rightarrow C=\dfrac{-1}{2}\left(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\right)\)
\(\Rightarrow C=\dfrac{-1}{2}\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{9.10}\right)\)
\(\Rightarrow C=\dfrac{-1}{2}\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)
\(\Rightarrow C=\dfrac{-1}{2}\left(1-\dfrac{1}{10}\right)\)
\(\Rightarrow C=\dfrac{-1}{2}.\dfrac{9}{10}\)
\(\Rightarrow C=\dfrac{-9}{20}\)
Tính bằng cách nhanh nhất
A = 4,8 . ( 3,1 - 1,5 ) + 1,5 . ( 4,8 - 3,1 )
A= 4,8. ( 3,1 - 1,5 )+ 1,5 . ( 4,8 - 3,1 )
=4,8 . 3,1 - 1,5 + 1,5 . 4,8 - 3,1
=4,8. ( 3,1 - 3,1 ) - ( 1,5 + 1,5 )
=4,8 .0 - 3
A= -3
0,2 x 17 x 7 + 0,14 x 520 + 3,1 x 14
tinh nhanh nha cac ban
chu y tra loi day du va chinh xac
0,2x17x7+0,14x520+3,1x14
(1,4x17)+(0,14x10x52)+(31:10x14)
(1,4x17)+(1,4x52)+(14:10x31)
(1,4x17)+(1,4x52)+(1,4x31)
1,4x(17+52+31)
1,4x100=140
tinh
3,15 + 5,55 - 3,1 + 1,3 - 3,9
3,15 + 5,55 - 3,1 + 1,3 - 3,9
= 8,7 - 3,1 + 1,3 - 3,9
= 5,6 + 1,3 - 3,9
= 6,9 - 3,9
= 3
Tính nhanh nếu có thể a. 5,3+(-0,7)-|-5,3| b. 5,3 -|-10| +|-3,1|-(-4,7) c.|-9|+|3,6| -|-4,1|+|1,3| d. 21/47+|-9/45|+|-26/47|+4/5 e. 3/8×58/3 -3/8×33×1/3
a: =5,3-0,7-5,3=-0,7
b: =5,3-10+3,1+4,7
=10-10+3,1=3,1
c: =9+3,6-4,1+1,3
=9+0,8=9,8
d: =21/47+26/47+1/5+4/5
=1+1
=2
e: =3/8(58/3-33/3)
=3/8*25/3
=25/8
tinh nhanh 21×72-11×72+90×72+49×125×16