Multiple choice

Let $'x'$ denotes the value of the product $(1+a+a^2+a^3+....\infty)(1+b+b^2+b^3+....\infty)$ where $'a'$ and $'b'$ are the roots of the quadratic equation $11x^2-4x-2=0$ and $'Y'$ denotes the numerical value of the infinite series $(log_b2)^0(log_b5^{4^0})+(log_b2)^1(log_b 5^{4^1})+(log_b2)^2(log_b5^{4^2})+(log_b2)^3(log_b5^{4^3})+.....\infty$ where $b=2000$ then the value of ($XY$) equals.

  1. $\displaystyle\dfrac{1}{5}$
  2. $\displaystyle\dfrac{13}{6}$
  3. $\displaystyle\dfrac{11}{15}$
  4. $\displaystyle\dfrac{22}{35}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The infinite product is (1/(1-a)) * (1/(1-b)) = 1/(1 - (a+b) + ab). Given 11x^2 - 4x - 2 = 0, a+b = 4/11 and ab = -2/11. Thus X = 1/(1 - 4/11 - 2/11) = 1/(5/11) = 11/5. For Y, the series is a geometric progression with first term log_b(5) and common ratio log_b(2)*4. Summing this leads to the result where XY = 11/15.