A = 1 + 3 + 32 + 33 + 34 + 35 + ... + 330
3A = 3 + 32 + 33 + 34 + 35 + 36 + ... + 331
3A - A =( 3 + 32 + 33 + 34 + 35 + 36 + ... + 331 ) - ( 1 + 3 + 32 + 33 + 34 + 35 + ... + 330 )
2A = 1 + 331
\(A=\frac{1+3^{31}}{2}\)
Vậy A = \(\frac{1+3^{31}}{2}\)
Nhớ K cho mk nha 
b )
B = 1 + x + x2 + x3 + x4 + ... + xn
xB = x + x2 + x3 + x4 + x5 + ... + xn+1
\(xB-B=\left(x+x^2+x^3+x^4+x^5+...+x^{n+1}\right)-\left(1+x+x^2+x^3+x^4+...+x^n\right)\) \(\left(x-1\right)B=x^{n+1}-1\)
\(B=\frac{x^{n+1}-1}{x}\)
Vậy B = \(\frac{x^{n+1}-1}{x}\)
