Đáp án:
$B=(7^2+1)(7^4+1)(7^8+1)(7^{16}+1)(7^{32}+1)$
$B=\frac{(7^2-1)(7^2+1)(7^4+1)(7^8+1)(7^{16}+1)(7^{32}+1)}{7^2-1}$
$B=\frac{((7^2)^2-1)(7^4+1)(7^8+1)(7^{16}+1)(7^{32}+1)}{7^2-1}$
$B=\frac{(7^4-1)(7^4+1)(7^8+1)(7^{16}+1)(7^{32}+1)}{7^2-1}$
$B=\frac{(7^8-1)(7^8+1)(7^{16}+1)(7^{32}+1)}{7^2-1}$
$B=\frac{(7^{16}-1)(7^{16}+1)(7^{32}+1)}{7^2-1}$
$B=\frac{(7^{32}-1)(7^{32}+1)}{7^2-1}$
$B=\frac{7^{64}-1}{7^2-1}$
