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a) \(\dfrac{\dfrac{3}{14}-\dfrac{3}{17}+\dfrac{3}{19}}{\dfrac{5}{19}+\dfrac{5}{14}-\dfrac{5}{17}}=\dfrac{3\left(\dfrac{1}{14}-\dfrac{1}{17}+\dfrac{1}{19}\right)}{5\left(\dfrac{1}{14}-\dfrac{1}{17}+\dfrac{1}{19}\right)}=\dfrac{3}{5}\)
c) \(\dfrac{\dfrac{2}{7}+\dfrac{2}{5}+\dfrac{2}{17}-\dfrac{2}{193}}{\dfrac{3}{7}+\dfrac{3}{5}+\dfrac{3}{17}-\dfrac{3}{293}}=\dfrac{2\left(\dfrac{1}{7}+\dfrac{1}{5}+\dfrac{1}{17}-\dfrac{1}{193}\right)}{3\left(\dfrac{1}{7}+\dfrac{1}{5}+\dfrac{1}{17}-\dfrac{1}{193}\right)}=\dfrac{2}{3}\)
d)
a: =35/17-18/17-9/5+4/5
=1-1=0
b: =-7/19(3/17+8/11-1)
=7/19*18/187=126/3553
c: =26/15-11/15-17/3-6/13
=1-6/13-17/3
=7/13-17/3=-200/39
Đặt A = 1.3 + 3.5 + 5.7 + ... + 15.17 + 17.19
6A = 1.3.6 + 3.5.6 + .... + 15.17.6 + 17.19.6
6A = 1.3.(5 + 1) + 3.5.(7 - 1) + ... + 15.17.(19 - 13) + 17.19.(21 - 15)
6A = 1.3.5 + 1.3 + 3.5.7 - 1.3.5 + ... + 15.17.19 - 13.15.17 + 17.19.21 - 15.17.19
6A = (1.3.5 + 3.5.7 + ... + 15.17.19 + 17.19.21) - ( 1.3.5 + ... + 13.15.17 + 15.17.19) + 1.3
6A = 17.19.21 + 1.3
6A = 6783 + 3
6A = 6786
A = 6780 : 6 = 1131
chu so tan cung o ve phai la 0 vi co so 10 tan cung la 0. ve trai tan cung la 5 vi 5nhan so le nao cung tan cung la 5 vay chu so tan cung la 0-5 =5
=1/1.3.5+1/3/5/7+1/5.7.9+......+1/17/19/21
=1/4.(5-1/1.3.5+7-3/3.5.7+.....+21-17/17/19/21
=1/4.(5/1.3.5-1/1.3.5+7/3.5.7-3/3.5.7+.....+21/17.19.21-17/17.19.21
=1/4.(1/1.3-1/3.5+1/3.5-1/5.7+.....+1/17.19-1/19.21)
=1/4.(1/3.1/21.17)
=1/4.3200/9603
= 800/9603
Chúc bạn học tốt^^
Đặt \(A=\frac{1}{1.3.5}+\frac{1}{3.5.7}+\frac{1}{5.7.9}+...+\frac{1}{17.19.21}\)
\(\Rightarrow4A=\frac{4}{1.3.5}+\frac{4}{3.5.7}+\frac{4}{5.7.9}+...+\frac{4}{17.19.21}\)
\(=\frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+\frac{1}{5.7}-\frac{1}{7.9}+...+\frac{1}{17.19}-\frac{1}{19.21}\)
\(=\frac{1}{1.3}-\frac{1}{19.21}=\frac{44}{133}\)
\(\Rightarrow A=\frac{44}{133}\div4=\frac{11}{133}\)
1+-3+5+-7+....+17+-19
=[1+(-3)]+[5+(-7)]+...+[17+(-19)]
= (-2)+(-2)+...+(-2) (co 5 so -2)
=-2.5=-10
moi nguoi an dung nha
đáp án = -10