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JEE Advanced 2016 Paper 1 · Mathematics · Quadratic Equations · medium

Let g(x)=x^3-6x^2+9x-2. The number of distinct real solutions of the equation |g(x)|=1 is

Numerical answer

Let g(x)=x3−6x2+9x−2g(x)=x^3-6x^2+9x-2. The number of distinct real solutions of the equation ∣g(x)∣=1|g(x)|=1 is
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Answer: 6

The equation splits into g(x)=1g(x)=1 and g(x)=−1g(x)=-1. Since g′(x)=3(x−1)(x−3)g'(x)=3(x-1)(x-3), gg has a local maximum g(1)=2g(1)=2 and a local minimum g(3)=−2g(3)=-2, and it rises from −∞-\infty to +∞+\infty. Any horizontal level strictly between −2-2 and 22 therefore meets the graph in exactly three points. Both levels 11 and −1-1 lie in this open interval, so each equation has three distinct real roots. The two equations cannot share a root because gg cannot equal 11 and −1-1 at once. Hence the total number of distinct solutions is 3+3=63+3=6. Numerically the roots are 0.468, 1.653, 3.8790.468,\,1.653,\,3.879 for g=1g=1 and 0.121, 2.347, 3.5320.121,\,2.347,\,3.532 for g=−1g=-1, all distinct, which confirms the count.
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