5 x 9 : 5 x 9 : 5 = 16 ,2 

tick cho minh nha

5+8=8+8

5x9:5x5

=45:5x5

=9x5

=45

108 nha Nguyễn Việt hoàng

3+5x9+5x12

=3+45+60

=48+60

=108

ko                                                      !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Lời giải:

$12A=1.5.12+5.9.(13-1)+9.13(17-5)+13.17(21-9)+....+77.81(85-73)+81.85(89-77)$

$=60+(5.9.13+9.13.17+13.17.21+...+77.81.85+81.85.89)-(1.5.9+5.9.13+9.13.17+...+73.77.81+77.81.85)$

$=60+81.85.89 - 1.5.9=612780$

A = 1.5 + 5.9 + 9.13 + ... + 81.85

A = \(\dfrac{12}{12}\)(1.5 + 5.9 + 9.13 + 81.85)

A = \(\dfrac{1}{12}\).(1.5.12 + 5.9.12.+ 9.13.12 + ...+ 81.85.12]

A = \(\dfrac{1}{12}\).[1.5.(9 + 3) + 5.9.(13 - 1) + 9.13.(17 - 5) +...+ 81.85.(89 - 77)]

^{A = \(\dfrac{1}{12}\).[1.5.9 + 1.3.5 + 5.9.13 - 5.9.1 + 9.13.17 - 9.13.5 + ...+ 81.85.89 - 81.85.77]}

A = \(\dfrac{1}{12}\).[1.3.5 + 81.85.89]

A = \(\dfrac{1}{12}\).[15 + 612765]

A = \(\dfrac{1}{12}\).612780

A = 51065

2015:5x9

=403x9

=3627

2015:5x9=403x9

               =3627

5 x 9 : 5 x 10 x 2 x 3 x 3 + 100 x 2

= ( 5 : 5 ) ( 9 x 10 x 2 x 3 x 3 ) + 100 x 2

= 1 x 1620 + 100 x 2

= 1620 + 200

= 1820

5 x 9 : 5 x 10 x 2 x 3 x 3 + 100 x 2

= ( 9 x 0 x 2 x 3 x 3 ) x ( 5 : 5 ) + 100 x 2

= 1620 x 1 + 200

= 1620 + 200

=1820

1 x 2 - 10 x 5 - 5 x 9

= 2 - 50 - 45

= 2 -( 50 + 45)

= 2 - 95

= -93 

\(1\times2-10\times5-5\times9\\ =2-\left(10-9\right)\times5\\ =2-1\times5\\ =2-5\\ =-3\)

Xem lại lớp.

\(B=\dfrac{1}{4}\times\left(\dfrac{4}{1\times5}+\dfrac{4}{5\times9}+\dfrac{4}{9\times13}+...+\dfrac{4}{125\times129}\right)\)

\(=\dfrac{1}{4}\times\left(1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+...+\dfrac{1}{125}-\dfrac{1}{129}\right)\)

\(=\dfrac{1}{4}\times\left(1-\dfrac{1}{129}\right)=\dfrac{1}{4}\times\dfrac{128}{129}=\dfrac{32}{129}\)

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