`C = 4 +4^2 +4^3 + ... +4^n`
`-> 4C = 4^2 + 4^3 + 4^4 + ... + 4^{n+1}`
`-> 4C - C = (4^2 + 4^3 + 4^4 + ... + 4^{n+1}) -(4 +4^2 +4^3 + ... +4^n)`
`-> 3C = 4^{n+1} - 4`
`-> C = {4^{n+1}-4}/3`
Đáp án:
Giải thích các bước giải:
C=`4+4^2+4^3+. . .+4^n`
4C=`4^2+4^3+4^4`+. . . +4^n+1
4C-C=(`4^2+4^3+4^4`+. . . +4^n+1)-(`4+4^2+4^3+. . .+4^n`)
3C =4^n+1-4
C =4^n+1-4/3
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