Let x be the smaller of the two solutions of the equation x2−4x+2=0. What are the first three digits after the decimal point in the (base 10) representation of the number x+x2+x3+⋯+x2009?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution:
The answer is 414. From the usual formula for solving quadratic equations, we get x=2−2. Now using the formula for the sum of a geometric progression, we have x+x2+x3+⋯+x2009=x(1+x+x2+⋯+x2008)=x1−x1−x2009=1−xx−1−xx2010. The second term of this sum is very small: indeed, with very loose estimates, we obtain (2−2)2010<(0.6)2010<((0.6)2)1005<210051<(210)1001<101001. On the other hand, the first term equals 1−xx=2−12−2=(2−1)(2+1)(2−2)(2+1)=12. Therefore the value of the expression is a quantity that differs from 2 by less than 10−100; the first digits of its decimal expansion will therefore be the same as those of 2, that is 1.414….
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