12x13
Đặt tính rồi tính:12x13
\(12\times13=156\)
nha bạn ^_^
Chiến Binh Ngân NgaTính nhanh:144+12x13-15x12
144 + 12 x 13 - 15 x 12
= 12 x 12 + 12 x 13 - 15 x 12
= 12 x ( 12 + 13 - 15 )
= 12 x 10
= 120
144+12x13-15x12
= 12x12+12x13-15x12
= 12x(12+13-15)
= 12x(25-15)=12x10=120
=144+12*(15-13)
=144+12*2
=144+24
=168
bài 1
12x13/5x24;25x17+25x12/29x13+29x14
câu 1: 12x13/5x24=12x13/5x12x2=13/5x2=13/10
câu 2 : 25x17+25x12/29x13+29x14
=25x(17+12)/29x(13+14)
=25.29/29.27
=25/27
5/10x11+ 5/11x12 + 5/12x13 +...+ 5/49x50
\(\frac{5}{10.11}+\frac{5}{11.12}+\frac{5}{12.13}+....+\frac{5}{49.50}\)
\(=5.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+.....+\frac{1}{49.50}\right)\)
\(=5.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+....+\frac{1}{49}-\frac{1}{50}\right)\)
\(=5.\left(\frac{1}{10}-\frac{1}{50}\right)\)
\(=5.\frac{2}{25}\)
\(=\frac{2}{5}\)
5/10 x 11 + 5/11 x 12 + 5/12 x 13 + .... + 5/49 x 50
= 5/10 - 5/11 + 5/11 - 5/12 + 5/12 - 5/13 + ....... + 5/49 - 5/50
= 5/10 - 5/50
= 2/5
Phần II. Tự luận (7 điểm)
Tính: a) 1 2 x 1 3 - 1 8
S = 1x2 + 2x3 + 3x4 + ……………… + 11x12 + 12x13
S = 1x2 + 2x3 + 3x4 + ……………… + 11x12 + 12x13
3S=1x2x3 + 2x3x3 + 3x4x3+ ………. + 11x12x3 + 12x13x3
Ta lấy K = 1x2x3 +2x3x4 + 3x4x5 + …… + 11x12x13 + 12x13x14
- 3S = 1x2x3 + 2x3x3 + 3x4x3+ ……… + 11x12x3 + 12x13x3
------------------------------------------------------------------------------------
K – 3S = 0 + 2x3x1 + 3x4x2 + …… .. + 11x12x10 + 12x13x11
K – 3S = K – 12x13x14
Từ đó suy ra: 3S = 12x13x14
S = 4x13x14 = 728
Cách 2:
S x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + …. + 11x12x(13-10) + 12x13x(14-11)
S x 3 = 1x2x3 + 2x3x4 – 2x3x1 + 3x4x5 – 3x4x2 + …..+ 11x12x13 – 11x12x10 +12x13x14 – 12x13x11
S x 3 = 12 x 13 x14
S = 4 x 13 x 14
S = 728
ai k minh minh k lai cho
C=7/10x11+7/11x12+7/12x13+.................+7/69x70 =?
C=7/10x11+7/11x12+7/12x13+.................+7/69x70
C=1x7/10x11+1x7/11x12+...........+1x7/69x70
C=7(1/10x11+1/11x12+1/12x13+....+1/69x70)
C=7(1/10-1/11+1/11-1/12+1/12-1/13+.......+1/69-1/70)
C=7(1/10-1/70)
C=7(7/70-1/70)
C=7x6/70
C=3/5
4/3x7+5/7x12+1/12x13+7/13x20+3/20x23=
Thực hiện phép tính
10x11+11x12+12x13+.....+29x30