tính = cách thuận tiện
a. 37x18-9x54+100
b. 94+87+81-71-77-84
c.2-4+6-8+10-12+14-16+18-20
d.1994x867+1995x133
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a) 1 - 2 + 3 - 4 + 5 - 6 + ... + 97 - 98 + 99 - 100 + 101 (có 101 số; 101 chia 2 dư 1)
= (1 - 2) + (3 - 4) + (5 - 6) + ... + (97 - 98) + (99 - 100) + 101 (có 50 nhóm và dư 1 số)
= -1 + (-1) + (-1) + ... + (-1) + (-1) + 101
= -1.50 + 101
= -50 + 101
= 51
2 câu c` lại cx chia thành nhóm r` lm tương tự
1 - 2 + 3 - 4 + ........ + 99 - 100 + 101
= ( 1 - 2) + ( 3 - 4 ) + ..... + ( 99 -100 ) + 101 ( 50 nhóm )
= - 1 + ( - 1 ) + ...... + ( -1) + 101 ( 50 số -1 )
= - 1 . 50 + 101
= 50 + 101
= 151
↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ ↑
a) \(\dfrac{5}{9}+\dfrac{13}{7}+\dfrac{15}{13}+\dfrac{8}{7}+\dfrac{4}{9}+\dfrac{11}{13}\)
\(=\left(\dfrac{5}{9}+\dfrac{4}{9}\right)+\left(\dfrac{13}{7}+\dfrac{8}{7}\right)+\left(\dfrac{15}{13}+\dfrac{11}{13}\right)\)
\(=1+3+2\)
\(=6\)
b) \(\dfrac{3}{5}+\dfrac{18}{9}+\dfrac{4}{10}\)
\(=0,6+2+0,4\)
\(=\left(0,6+0,4\right)+2\)
\(=1+2\)
\(=3\)
\(a\dfrac{5}{9}+\dfrac{4}{9}+\dfrac{13}{7}+\dfrac{8}{7}+\dfrac{15}{13}+\dfrac{11}{13}=\dfrac{9}{9}+\dfrac{21}{7}+\dfrac{26}{13}=1+3+2=6\)
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20
=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+20+10
=20+20+20+20+20+20+20+20+20+20+10
=20×10+10
=200+10
=210
= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) + (4 + 16 ) + ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12) + ( 9+ 11 ) + 20
= 20 + 20 + 20 +20 + 20 +20 + 20 + 20 + 20 +20
= 20 x 10
= 200
tui đầu tiên đó
`a)1/10 + 2/10 + 3/10 + 4/10 + 5/10 + 6/10 + 7/10 + 8/10 + 9/10`
`=(1/10+9/10)+(2/10+8/10)+(3/10+7/10)+(4/10+6/10)+5/10`
`=10/10 + 10/10+10/10+10/10+1/2`
`=1+1+1+1+1/2`
`=2+2+1/2`
`=4+1/2`
`=8/2+1/2`
`=9/2`
__
`13,25:0,5+13,25:0,25+13,25:0,125+13,25×6`
`=13,25×1/(0,5)+13,25×1/(0,25)+13,25×1/(0,125)+13,25×6`
`=13,25×(1/(0,5)+1/(0,25)+1/(0,125)+6)`
`=13,25×(2+4+8+6)`
`=13,25×20`
`=265`
`#QiN`
a: =12h-7h=5h
b: =1,4h*1+1,4h+1,4h*8
=1.4h*10
=14h
c: =1,25h+1,25h+1,25h*8
=1,25h*10=12,5h
a) 37 x 18 - 9 x 54 + 100
= 37 x 2 x 9 - 9 x 54 + 100
= 74 x 9 - 8 x 54 + 100
= ( 74 - 54 ) x 9 + 100
= 20 x 9 + 20 x 5
= 20 x ( 9 + 5 )
= 20 x 14
= 280
b) 94 + 87 + 81 - 71 - 77 - 84
= ( 94 - 84 ) + ( 87 - 77 ) + ( 81 - 71 )
= 10 + 10 + 10
= 10 x 3
= 30