Đặt tổng trên = A
\(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{152}\)
\(Ax2=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{76}\)
\(Ax2-A=\left(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{76}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{152}\right)\)
\(A=1-\frac{1}{152}\)
\(A=\frac{151}{152}\)
\(\text{Đặt tổng trên = A}\)
\(A=\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{152}\)
\(Ax2=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{76}\)
\(Ax2-A=\left(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{76}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{16}+...+\frac{1}{152}\right)\)
\(A=1-\frac{1}{152}\)
$A=\frac{151}{152}$
tính nhanh p/s 1+ 5/4 + 5/8 + 5/16 + 5/32 + 5/64
