`A = (2^10 * 3^10 -2^10 *3^9)/(2^9 * 3^10)`
`=> A = ((2^10 * 3^9)(1*3 -1))/(2^9 * 3^10)`
`=> A = (2*2)/3`
`=> A = 4/3`
Vậy `A = 4/3`
`B = 4/(11 *16) + 4/(16*21) + 4/(21*26) + ... + 4/(61*66)`
`=> B = 4*(1/(11*16) + 1/(16*21) + ... +1/(61*66) )`
Đặt `A = 1/(11*16) + 1/(16*21) + ... +1/(61*66)`
`=> 5A = 5/(11*16) + 5/(16*21) + ... +5/(61*66)`
`=> 5A = 1/11 - 1/16 + 1/16 - 1/21 + ... + 1/61 - 1/66`
`=> 5A = 1/11 - 1/66`
`=> 5A = 5/66`
`=> A = 5/66 :5`
`=> A = 1/66`
`=> B = 4*A`
`=> B = 4 * 1/66 = 4/66 = 2/33`
Vậy `B = 2/33`
`c) 1-(2x + 5) =12`
`=> 2x + 5 = 1 -12`
`=> 2x + 5 = -11`
`=> 2x = -11 - 5`
`=> 2x = -16`
`=> x = -16 :2`
`=> x =-8`
Vậy `x =-8`
a: \(A=\frac{2^{10}\cdot3^{10}-2^{10}\cdot3^9}{2^9\cdot3^{10}}\)
\(=\frac{2^{10}\cdot3^9\left(3-1\right)}{2^9\cdot3^{10}}=\frac23\cdot2=\frac43\)
b: \(B=\frac{4}{11\cdot16}+\frac{4}{16\cdot21}+\cdots+\frac{4}{61\cdot66}\)
\(=\frac45\left(\frac{5}{11\cdot16}+\frac{5}{16\cdot21}+\cdots+\frac{5}{61\cdot66}\right)\)
\(=\frac45\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\cdots+\frac{1}{61}-\frac{1}{66}\right)\)
\(=\frac45\left(\frac{1}{11}-\frac{1}{66}\right)=\frac45\cdot\frac{5}{66}=\frac{4}{66}=\frac{2}{33}\)
c: 1-(2x+5)=12
=>2x+5=1-12=-11
=>2x=-11-5=-16
=>\(x=-\frac{16}{2}=-8\)
