bai 4: tinh bang cach thuan tien nhat: a, 5/9*5/2+4/9*5/2 , b, 12*3/4-3/4*12/7 lam kieu gi
giup minh voi
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\(\dfrac{12}{7}\) \(\times\) \(\dfrac{3}{4}\) - \(\dfrac{3}{4}\) \(\times\) \(\dfrac{12}{7}\)
= \(\dfrac{9}{7}\) - \(\dfrac{9}{7}\)
= 0
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
= (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) +5
= 10 + 10 + 10 +10 + 5
= 10 x 4 + 5
= 40 + 5
= 45
1+2+3+4+5+6+7+8+9:9
=1+2+3+4+5+6+7+8+1
=(1+1)+(2+8)+(3+7)+(4+6)+5
=2+10+10+10+5
=(2+5)+(10+10+10)
=7+30
=37
a) = \(\frac{1+2+3+..+11}{3}=\frac{\left(1+11\right)+\left(2+9\right)+..+\left(5+7\right)+6}{3}=\frac{12.5+6}{3}=\frac{66}{3}=22\)
b) A = \(9+\frac{9}{2}+\frac{9}{4}+\frac{9}{8}+...+\frac{9}{128}+\frac{9}{256}\)
2 x A = 18 + 9 + \(\frac{9}{2}+\frac{9}{4}+\frac{9}{8}+...+\frac{9}{128}\)
Lấy 2 x A - A = 18 - \(\frac{9}{256}\)
A = 4599/256
7/9 x 5/16 : 5/9 x4/7
=7/9 x 5/16 x9/5 x 4/7
=7/9 x 45/80 x 4/7
=7/9 x 9/16 x 4/7
=7 x 9 x 4 /9 x 16 x 7
=1/4
\(a,\dfrac{5}{9}\times\dfrac{5}{2}+\dfrac{4}{9}\times\dfrac{5}{2}=\dfrac{5}{2}\times\left(\dfrac{5}{9}+\dfrac{4}{9}\right)=\dfrac{5}{2}\times1=\dfrac{5}{2}\)
\(12\times\dfrac{3}{4}-\dfrac{3}{4}\times\dfrac{12}{7}=\dfrac{3}{4}\times\left(12-\dfrac{12}{7}\right)=\dfrac{3}{4}\times\dfrac{72}{7}=\dfrac{54}{7}\)