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1 + 3 + 5 + 7 + ... + x = 25
Số số hạng : (x-1) : 2 + 1 = (x+1)/2
Tổng : \(\frac{x+1}{2}\cdot\frac{x+1}{2}=25\)
\(\frac{x+1}{2}\cdot\frac{x+1}{2}=5\cdot5\)
\(\frac{x+1}{2}=5\)
\(x+1=10\)
\(x=9\)
4 + x = 7 - 2x
4 + x + 2x = 7
4 + 3x = 7
3x = 7 - 4
3x = 3
x = 3 : 3
x =1
5 + x = 8,3 + 4,7 - x
5 + x + x = 8,3 + 4,7
5 + 2x = 13
2x = 13 - 5
2x = 8
x = 8 : 2
x= 4
câu 1 = 0
câu 2 : X=\(\frac{16}{105}\)
X =\(\frac{180}{77}\)
X = 63
(18\(\dfrac{1}{2}+5\dfrac{3}{8}-7\dfrac{5}{24}\)) - 16\(\dfrac{2}{3}\)
=( \(\dfrac{37}{2}\) + \(\dfrac{43}{8}\) - \(\dfrac{171}{24}\) ) - \(\dfrac{50}{3}\)
= \(\dfrac{50}{3}\) - \(\dfrac{50}{3}\)
=0
a)=(3/8+10/16)+(7/12+10/24)
=1+1=2
c)=(4/6+14/6)+(7/13+19/13)+(17/9+1/9)
=3+2+2=7
a) 3/5x7/8:10/9=21/40:10/9=189/400
b) 3 2/7x2 1/4:1 2/5=23/7x9/4:7/5=207/28:7/5=1035/196
c) 6/9+24/15+12/9+6/15=34/15+12/9+6/15=18/5+6/15=4
d) 3 2/9-( 1 5/8+1 2/9 )=29/9-(13/8+11/9)=29/9-205/72=3/8
Bài 2:
a, \(\dfrac{5}{23}\) \(\times\) \(\dfrac{17}{26}\) + \(\dfrac{5}{23}\) \(\times\) \(\dfrac{9}{26}\)
= \(\dfrac{5}{23}\) \(\times\) ( \(\dfrac{17}{26}\) + \(\dfrac{9}{26}\))
= \(\dfrac{5}{23}\) \(\times\) \(\dfrac{26}{26}\)
= \(\dfrac{5}{23}\)
b, \(\dfrac{3}{4}\) \(\times\) \(\dfrac{7}{9}\) + \(\dfrac{7}{4}\) \(\times\) \(\dfrac{3}{9}\)
= \(\dfrac{7}{12}\) + \(\dfrac{7}{12}\)
= \(\dfrac{14}{12}\)
= \(\dfrac{7}{6}\)