Problem: Determine the number of real roots of the equation x8−x7+2x6−2x5+3x4−3x3+4x2−4x+25=0
Solution
Solution: Write x8−x7+2x6−2x5+3x4−3x3+4x2−4x+25=x(x−1)(x6+2x4+3x2+4)+25 If x(x−1)≥0, i.e. x≤0 or x≥1, the equation has no roots. If 0<x<1, then 0>x(x−1)=(x−21)2−41≥−41 and x6+2x4+3x+4<1+2+3+4=10. The value of the left-hand side of the equation now is larger than −41⋅10+25=0. The equation has no roots in the interval (0,1) either.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.