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A = 7 + 72 + 73 + .........+ 7n-1 + 7n
7A = 72 + 73 +..........+ 7n-1 + 7n + 7n-1
7A - A = 7n+1 - 7
6A = ( 7n+1 - 7)
A = (7n+1 - 7) : 6
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\(\dfrac{-7^n}{-7^{n-1}}=\dfrac{-7^n}{-7^n:\left(-7\right)}=\dfrac{1}{\dfrac{1}{-7}}=-7\)
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\(7^n+7^{n+1}+7^{n+2}=19551\)
\(7^n.\left(1+7+7^2\right)=19551\)
\(7^n=19551:57=343=7^3\)
n = 3
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\(\dfrac{\left(-7\right)^n}{\left(-7\right)^{n-1}}=\dfrac{\left(-7\right)^n}{\left(-7\right)^n\cdot\left(-7\right)^{-1}}=\dfrac{1}{\left(-7\right)^{-1}}=-7\)
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(-7)n
(-7)n-1
nha