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=( 9 + 13 +....+85 ) - ( x + x +.....+ x) =925
=940 - x.20 = 925
x.20 = 940-925
x.20 = 15
x = 15:20
x = 0.75
a)\(\left(5^{23}.5^{18}\right):5^{39}-5^0.0^5\)
= \(5^{41}:5^{39}-1.0\)
= \(5^2-0\)
= \(25-0=25\)
b)\(\left[2.3^3+144:\left(7^2-925:5^2\right)\right].\left|-8\right|\)
= \(\left[2.27+144:\left(49-925:25\right)\right].8\)
= \(\left[54+144:\left(49-37\right)\right].8\)
= \(\left[54+144:12\right].8\)
= \(\left[54+12\right].8\)
= \(66.8\)
= 528
1, \(\frac{5}{21}+0,5-\frac{19}{23}+\frac{16}{21}-\frac{4}{23}\)
\(=\frac{5+16}{21}-\frac{19+4}{23}+0,5\)
\(=1-1+0,5\)
\(=0,5\)
2,\(\frac{27}{23}+\frac{5}{21}-\frac{4}{23}+\frac{6}{21}+\frac{1}{2}\)
\(=\frac{27-4}{23}+\frac{5+6}{21}+\frac{1}{2}\)
\(=1+\frac{11}{21}+\frac{1}{2}\)
\(=\frac{42+22+21}{42}\)
\(=\frac{85}{42}\)
\(\dfrac{16}{21}+\dfrac{4}{23}+\dfrac{5}{21}-\dfrac{4}{23}-\dfrac{5}{21}\)
\(=\left(\dfrac{16}{21}+\dfrac{5}{21}-\dfrac{5}{21}\right)+\left(\dfrac{4}{23}-\dfrac{4}{23}\right)\)
\(=\dfrac{16}{21}+0+0\)
\(=\dfrac{16}{21}\)
\(\dfrac{16}{21}+\dfrac{4}{23}+\dfrac{5}{21}-\dfrac{4}{23}-\dfrac{5}{21}\)
\(=\left(\dfrac{16}{21}+\dfrac{5}{21}-\dfrac{5}{21}\right)+\left(\dfrac{4}{23}-\dfrac{4}{23}\right)\)
\(=\left(\dfrac{16+5-5}{21}\right)+\left(\dfrac{4-4}{23}\right)\)
\(=\dfrac{16}{21}+0\)
\(=\dfrac{16}{21}\)
- ( 12 + 21 - 23 ) - ( 23 - 21 + 10 )
= (- 10) - 12
= -22
Số lượng số hạng là:
\(\left(925-21\right):2+1=453\) (số hạng)
Tổng của dãy số là:
\(\left(925+21\right)\times453:2=214269\)
\(21+23+...925=\left(\left(925-21\right):2+1\right)\left(21+95\right)=453x116=52548\)