Ta có:
A=1+3+32+.....+320
3A=3+32+33+.......+321
3A-A=3+32+33+......+321-1-3-32-......-320
2A=321-1
A=\(\frac{3^{21}-1}{2}\)
\(\Rightarrow B-A=\frac{3^{21}}{2}-\frac{3^{21}-1}{2}\)
\(\Rightarrow B-A=\frac{3^{21}-3^{21}-1}{2}\)
\(\Rightarrow B-A=-\frac{1}{2}\)
\(\Rightarrow B-A=-0,5\)
Ta có :
A = 1 + 3 + 32 + 33 + ... + 320
3A = 3 + 32 + 33 + 34 + ... + 321
3A - A = (3 + 32 + 33 + ... + 321) - (1 + 3 + 32 + 33 + ... + 320)
2A = 321 - 1
A = \(\frac{3^{21}-1}{2}\)
=> B - A = \(\frac{3^{21}}{2}-\frac{3^{21}-1}{2}\)
\(B-A=\frac{3^{21}-3^{21}-1}{2}\)
\(B-A=-\frac{1}{2}\)
\(B-A=-0,5\)
Ủng hộ mk nha !!! ^_^
Ta có :
A = 1 + 3 + 32 + 33 + ... + 320
3A = 3 + 32 + 33 + 34 + ... + 321
3A - A = (3 + 32 + 33 + ... + 321) - (1 + 3 + 32 + 33 + ... + 320)
2A = 321 - 1
A = $\frac{3^{21}-1}{2}$321−12
=> B - A = $\frac{3^{21}}{2}-\frac{3^{21}-1}{2}$3212 −321−12
$B-A=\frac{3^{21}-3^{21}-1}{2}$B−A=321−321−12
$B-A=-\frac{1}{2}$B−A=−12
