[Toán 6] Tính

C

chaugiang81

$3A= 3^2 +3^3 +...+3^{21}$
$=>3A- A= 3^2 + 3^3 + ...+3^{21} - 3 - 3^2 -...- 3^{20}$
$<=> 2A= 3^{21} - 3$
$=>A= \dfrac{3^{21}-3}{2}$
 
C

chi254


$A=3+3^2+3^3+3^4+.............+3^{18}+ 3^{19}+3^{20}$

\Rightarrow$3A = 3^2+3^3+3^4+...........+3^{18}+ 3^{19}+3^{20}+3^{21}$

\Rightarrow$3A - A = 3^{21} - 3$

\Leftrightarrow$2A = 3^{21} - 3$

\Rightarrow$A = \frac{3^{21} - 3}{2}$
 
Last edited by a moderator:
Top Bottom