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a.A=-1/2-1/6-1/12-1/20-1/30-1/42-1/56-1/72=-151/180
Vậy A=151/180
b.B=0,5+0,4+1/3+1/6+5/7-4/35=2
Vậy B=2
A = 1/2 + 1/4 + 1/8 + ... + 1/128
A = 1/2^1 + 1/2^2 + 1/2^3 + ... + 1/2^7
2A = 1 + 1/2 + 1/2^2 + ... + 1/2^6
2A - A = 1 - 1/2^7 = A
\(\dfrac{1}{12345}-3\dfrac{3}{4}+0,125+3\dfrac{3}{4}-\dfrac{1}{8}-\dfrac{1}{12345}\)
\(=\left(\dfrac{1}{12345}-\dfrac{1}{12345}\right)+\left(-3\dfrac{3}{4}+3\dfrac{3}{4}\right)+\left(0,125-\dfrac{1}{8}\right)\)
\(=0+0+\left(\dfrac{1}{8}-\dfrac{1}{8}\right)\)
\(=0\)
#Toru
Ta có :
\(S=\left(\frac{1}{2}+\frac{3}{4}+\frac{7}{8}+\frac{15}{16}+\frac{31}{32}+\frac{63}{64}+\frac{127}{128}\right)-6\)
\(S=\left(\frac{64}{128}+\frac{102}{128}+\frac{112}{128}+\frac{120}{128}+\frac{124}{128}+\frac{126}{128}+\frac{127}{128}\right)-6\)
\(S=\frac{64+102+112+120+124+126+127}{128}-6\)
\(S=\frac{775}{128}-6\)
\(S=\frac{775}{128}-\frac{768}{128}\)
\(S=\frac{7}{128}\)
S=1/2+3/4+7/8+15/16+31/32+63/64+127/128 -6
S= 1-1/2 + 1-1/4 + 1-1/8 + 1-1/16 + 1-1/32 + 1-1/64+ 1-1/128 - 6
S= (1+1+1+1+1+1+1-6)- (1/2+1/4+1/8+1/16 + 1/32+1/64+1/128)
S= 1- 111/128
S= 17/128
(Làm lụi nha bn)