13.
a, A = [TEX](5 + 1)(5^2 + 1)(5^4 + 1)(5^8 + 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
4A = [TEX](5 - 1)(5 + 1)(5^2 + 1)(5^4 + 1)(5^8 + 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
= [TEX](5^2 - 1)(5^2 + 1)(5^4 + 1)(5^8 + 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
= [TEX](5^4 - 1)(5^4 + 1)(5^8 + 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
= [TEX](5^8 - 1)(5^8 + 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
= [TEX](5^{16} - 1)(5^{16} + 1)(5^{32} + 1)[/TEX]
= [TEX](5^{32} - 1)(5^{32} + 1)[/TEX]
= [TEX]5^{64} - 1[/TEX]
=> A = [TEX]\frac{5^{64} - 1}{4}[/TEX]
14.
A = [TEX]x^2 + 2xy + y^2 - 4x - 4y + 1[/TEX]
= [TEX](x + y)^2 - 4(x + y) + 1[/TEX]
Thay x + y = 5 vào rồi tính