Bài 1: Cho A=\(2+2^2+2^3+...+2^{60}\) CMR: \(A⋮3\) \(A⋮7\) \(A⋮15\)
Bài 2: Cho B=\(\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2+\left(\dfrac{1}{2}\right)^3+...+\left(\dfrac{1}{2}\right)^{98}+\left(\dfrac{1}{2}\right)^{99}\)
CMR: B<1
Bài 3: Tìm a,b,c\(\in Q\) biết: \(a.b=2\) ; \(b.c=3\) ; \(c.a=54\)