## 1,tính hợp lí a,8/7×3/4×7/8×12/15×2 b,(98/99+99/100+100/101)x(1/12-1/3+1/4) 2,tính nhanh S=1/6+1/12+1/20+1/30+1/42+1/56 3,tính S=1/1x2x3+1/2x3x4+1/

Question

1,tính hợp lí
a,8/7×3/4×7/8×12/15×2
b,(98/99+99/100+100/101)x(1/12-1/3+1/4)
2,tính nhanh
S=1/6+1/12+1/20+1/30+1/42+1/56
3,tính
S=1/1x2x3+1/2x3x4+1/3x4x5+…………+1/98x99x100
làm nhanh mình đang cần gấp nhé

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1 giờ 2021-09-07T12:34:07+00:00 2 Answers 0 views 0

1. Đáp án:

Giải thích các bước giải:

a,8/7.(3/4).(7/8).(12/15).2

=(8/7).(7/8).(3/4.(12/15).2

=1.2.(3/4).(4/5)

=2.(3/5)=6/5

b,(98/99+99/100+100/101).(1/12-1/3+1/4)

=(98/99+99/100+100/101).(1/12-1/12)

=(98/99+99/100+100/101).0=0

2,

S=1/6+1/12+1/20+1/30+1/42+1/56

=1/(2.3)+1/(3,4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)

=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

=1/2-1/8=3/8

S=1/(1.2.3)+1/(2.3.4)+1/(3.4.5)+…………+1/(98.99.100)

2S=2/(1.2.3)+2/(2.3.4)+2/(3.4.5)+…………+2/(98.99.100)

2S=1/(1.2)-1/(2.3)+1/(2.3)-1/(3.4)+1/(3.4)-1/(4.5)+…….+1/(98.99)-1/(99.100)

2S=1/2-1/9900=4949/9900

=>S=4949/19800

dấu . là dấu nhân nhé

2. Đáp án:

Giải thích các bước giải:

Bài 1:

a, 8/7 x 3/4 x 7/8 x 12/15 x 2

= (8/7 x 7/8) x (3/4 x 12/15) x 2

= 1 x 1 x 2

= 2

b, (98/99+99/100+100/101) x (1/12-1/3+1/4)

= (98/99+99/100+100/101) x (1/12 – 4/12 + 3/12)

= (98/99+99/100+100/101) x 0

= 0

Bài 2:

S= 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56

= 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8

= 1/2 – 1/3 + 1/3 – 1/4 + 1/4 – 1/5 + 1/5 – 1/6 + 1/6 – 1/7 + 1/7 – 1/8

= 1/2 – 1/8

= 4/8 – 1/8

= 3/8

Vậy S = 3/8

Bài 3:

S=1/1x2x3+1/2x3x4+1/3x4x5+…………+1/98x99x100

4S = 1/1x2x3x4 + 1/2x3x4x(5-1) + ……+1/98x99x100x(101-97)

4S =  1/1x2x3x4  + 1/2x3x4x5 – 1/1x2x3x4+ …. 1/98x99x100x101- 1/97x98x99x1004

S =  1/98x99x100x101