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a: \(A=\left(\dfrac{1}{99}+1\right)+\left(\dfrac{2}{98}+1\right)+...+\left(\dfrac{98}{2}+1\right)+1\)
\(=\dfrac{100}{99}+\dfrac{100}{98}+...+\dfrac{100}{2}+\dfrac{100}{100}\)
\(=100\cdot\left(\dfrac{1}{2}+\dfrac{1}{3}+...+\dfrac{1}{100}\right)\)=100B
=>B/A=1/100
b: \(A=\left(\dfrac{1}{49}+1\right)+\left(\dfrac{2}{48}+1\right)+\left(\dfrac{3}{47}+1\right)+...+\left(\dfrac{48}{2}+1\right)+\left(1\right)\)
\(=\dfrac{50}{49}+\dfrac{50}{48}+....+\dfrac{50}{2}+\dfrac{50}{50}\)
\(=50\left(\dfrac{1}{2}+\dfrac{1}{3}+...+\dfrac{1}{50}\right)\)
\(B=\dfrac{2}{2}+\dfrac{2}{3}+\dfrac{2}{4}+...+\dfrac{2}{49}+\dfrac{2}{50}\)
\(=2\left(\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...+\dfrac{1}{49}+\dfrac{1}{50}\right)\)
=>A/B=25
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\(3A=3\left(-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{50}}-\frac{1}{3^{51}}\right)\)
\(=-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{49}}-\frac{1}{50}\)
=> \(3A+A=\left(-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{49}}-\frac{1}{3^{50}}\right)+\left(-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{50}}-\frac{1}{3^{51}}\right)\)
\(4A=-1-\frac{1}{3^{51}}\)
\(A=-\frac{1}{4}\left(1+\frac{1}{5^{51}}\right)=-\frac{5^{51}+1}{4.5^{51}}\)
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A = \(\frac{1}{1.2}+\frac{1}{3.4}+\frac{1}{5.6}+...+\frac{1}{49.50}\)
A = \(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{49}-\frac{1}{50}\)
A = \(\left(1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{49}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{50}\right)\)
A = \(\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+...+\frac{1}{49}+\frac{1}{50}\right)-2.\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{50}\right)\)
A = B - 2C ( ĐPCM )
Vậy A = B - 2C
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Câu 1
Ta có: 2x = 3y => x/3 = y/2 => x/21 = y/14(chia 2 vế với 7) (1)
Tacó: 5x = 7z => x/7 = z/5 => x/21 =z/15 (chia 2 vế với 3) (2)
Từ (1) và (2) ta có: x/21 = y/14 = z/15= 3x/63 = 7y/98 = 5z/75
Áp dụng t/c của dãy tỉ số = nhau, ta có:
x/21 = y/14 = z/15 = 3x/63 = 7y/98 = 5z/75 = (3x-7y+5z)/(63 -98+75) = 30/40=3/4
Khi đó: x/21 = 3/4 => x =63/4
y/14 = 3/4 => y = 21/2
z/15 = 3/4=> z = 45/24
Vậy x =63/4; y = 21/2; z = 45/24
Câu 2:
a) A= 1/3.5+1/5.7+..........+1/97.99
2A= 2/3.5 + 2/5.7 +...+ 2/97.99
2A=(1/3-1/5)+(1/5-1/7)+...+(1/97-1/99)
2A=1/3-1/99
2A= 32/99
A= 32/99 :2
A= 16/99
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