55+55=?
Cách viết nào sau đây là đúng:
A. /-55/ = -55 B. /-55/ = 55 C. -/-55/ = 55 D. -/55/ = 55
55*55=55*5+55+55*.....
55/11x16 + 55/16x21 + 55/21x26 + 55/26x31 + 55/31x36 + 55/36x41 = ?
Ta có:
55/11x16+55/16x21+55/21x26x.....x55/36x41
=55/5 x (1/11-1/16+1/16-1/21+.......+1/36-1/41)
=55/5 x (1/11-1/41)
=55/5x30/451
=30/41
nhớ ****
A=55/11.16+55/16.21+55/21.26+55/26.31+55/31.36+55/36.41 = ?
A=55/11.16+55/16.21+55/21.26+55/26.31+55/31.36+55/36.41
=11.(5/11.16+5/16.21+...+5/36.41)
=11.(1/11-1/16+1/16-1/21+...+1/36-1/41)
=11.(1/11-1/41)
=11.30/451
=30/41
55 * 5 +55+55* ..... = 55*55
= 275 + 55 + 55 x .... = 3025
= 330 + 55 x .... = 3025
55 x ... = 3025 -330
55 x ... = 29695
....= 29695 : 55
..... = 539,9.... ( nếu là toán lớp 4 thì đề sai nhé )
55/11*16 +55/16*21 +55/21*26 +55/26*31+55/31*36 +55/36*41
Lời giải:
Gọi tổng trên là $A$.
$A=11\times (\frac{5}{11\times 16}+\frac{5}{16\times 21}+\frac{5}{21\times 26}+....+\frac{5}{36\times 41})$
$=11\times (\frac{16-11}{11\times 16}+\frac{21-16}{16\times 21}+\frac{26-21}{21\times 26}+....+\frac{41-36}{36\times 41})$
$=11\times (\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+...+\frac{1}{36}-\frac{1}{41})$
$=11\times (\frac{1}{11}-\frac{1}{41})=1-\frac{11}{41}=\frac{30}{41}$
C= 55/6x11 + 55/11x16 + 55/16x21 55/21x26 55/ 26x31
\(=11\left(\dfrac{5}{6\cdot11}+\dfrac{5}{11\cdot16}+\dfrac{5}{16\cdot21}+\dfrac{5}{21\cdot26}+\dfrac{5}{26\cdot31}\right)\)
=11(1/6-1/11+1/11-1/16+1/16-1/21+1/21-1/26+1/26-1/31)
=11*25/186=275/186
C= 55/6x11 + 55/11x16 + 55/16x21 55/21x26 55/ 26x31
\(\dfrac{55}{6x11}+\dfrac{55}{11x16}+\dfrac{55}{21x16}+\dfrac{55}{21x26}+\dfrac{55}{31x26}\)
\(=55\left(\dfrac{1}{6x11}+\dfrac{1}{11x16}+\dfrac{1}{21x16}+\dfrac{1}{21x26}+\dfrac{1}{31x26}\right)\)
\(=55\left(\dfrac{1}{5}x\left(\dfrac{1}{6}-\dfrac{1}{11}\right)+\dfrac{1}{5}x\left(\dfrac{1}{11}-\dfrac{1}{16}\right)+\dfrac{1}{5}x\left(\dfrac{1}{16}-\dfrac{1}{21}\right)+\dfrac{1}{5}x\left(\dfrac{1}{21}-\dfrac{1}{26}\right)+\dfrac{1}{5}x\left(\dfrac{1}{26}-\dfrac{1}{31}\right)\right)\)
\(=55x\dfrac{1}{5}x\left(\dfrac{1}{6}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{31}\right)\)
\(=11x\left(\dfrac{1}{6}-\dfrac{1}{31}\right)=11x\left(\dfrac{31-6}{186}\right)=11x\dfrac{25}{186}=\dfrac{275}{186}\)
C=\(\dfrac{55}{6\cdot11}\)+\(\dfrac{55}{11\cdot16}\)+\(\dfrac{55}{16\cdot21}\)+\(\dfrac{55}{21\cdot26}\)+\(\dfrac{55}{26\cdot31}\)
=\(\dfrac{1}{6}\)-\(\dfrac{1}{11}\)+\(\dfrac{1}{11}\)-\(\dfrac{1}{16}\)+\(\dfrac{1}{16}\)-\(\dfrac{1}{21}\)+\(\dfrac{1}{21}\)-\(\dfrac{1}{26}\)+\(\dfrac{1}{26}\)-\(\dfrac{1}{31}\)
=\(\dfrac{1}{6}\)-\(\dfrac{1}{31}\)=\(\dfrac{25}{186}\)
\(C=\dfrac{55}{6\times11}+\dfrac{55}{11\times16}+\dfrac{55}{16\times21}+\dfrac{55}{21\times26}+\dfrac{55}{26\times31}\)
\(\dfrac{1}{11}C=\dfrac{5}{6\times11}+\dfrac{5}{11\times16}+\dfrac{5}{16\times21}+\dfrac{5}{21\times26}+\dfrac{5}{26\times31}\)
\(\dfrac{1}{11}C=\dfrac{1}{6}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{31}\)
\(\dfrac{1}{11}C=\dfrac{1}{6}-\dfrac{1}{31}=\dfrac{25}{186}\)
\(C=\dfrac{25}{186}:\dfrac{1}{11}=\dfrac{275}{186}\)
tinh nhanh
A = 55/11x16 + 55/16x21 + 55/21x26 + 55/26x31 + 55/31x36 + 55/36x41
\(\frac{A}{11}=\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+\frac{5}{26.31}+\frac{5}{31.36}+\frac{5}{36.41}\)
\(\frac{A}{11}=\frac{16-11}{11.16}+\frac{21-16}{16.21}+\frac{26-21}{21.26}+...+\frac{41-36}{36.41}\)
\(\frac{A}{11}=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+...+\frac{1}{36}-\frac{1}{41}\)
\(\frac{A}{11}=\frac{1}{11}-\frac{1}{41}=\frac{30}{11.41}\Rightarrow A=\frac{11.30}{11.41}=\frac{30}{41}\)
\(\sqrt[1]{\dfrac{2}{3}_3\dfrac{1}{2}3_3\dfrac{3}{4}^3}\)
A = \(\dfrac{55}{11x16}\)+ \(\dfrac{55}{16x21}\)+\(\dfrac{55}{21x26}\)+\(\dfrac{55}{26x31}\)+\(\dfrac{55}{31x36}\)+\(\dfrac{55}{36x41}\)
`A = 55/( 11xx16 ) + 55/( 16xx21 ) + 55/( 21xx26 ) + 55/( 26xx31) + 55/( 31xx36)+55/(36xx41)`
`A = 11 xx ( 5/( 11xx16 ) + 5/( 16xx21 ) + 5/( 21xx26 ) + 5/( 26xx31) + 5/( 31xx36)+5/(36xx41)`
`A = 11 xx ( 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 + 1/26 - 1/31 + 1/31 - 1/36 + 1/36 - 1/41 )`
`A = 11 xx ( 1/11 - 1/41 )`
`A = 1 - 11/41`
`A = 30/41`
\(A=55.\left(\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{31}+\dfrac{1}{31}-\dfrac{1}{36}+\dfrac{1}{36}-\dfrac{1}{41}\right)\)
\(A=55\times\left(\dfrac{1}{11}-\dfrac{1}{41}\right)=55\times\dfrac{30}{451}=\dfrac{150}{41}\)