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Application of Derivatives question

2021 · 26 Feb · Shift 2 · Q47
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  5. /2021 · 26 Feb · Shift 2 · Q47

Application of Derivatives question

2021 · 26 Feb · Shift 2 · Q47

JEE MainMathematicsApplication of DerivativesNumerical+4 / −1
Let a be an integer such that all the real roots of the polynomial 2x5 + 5x4 + 10x3 + 10x2 + 10x + 10 lie in the interval (a, a + 1). Then, |a| is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 2

  1. Let P(x)=2x5+5x4+10x3+10x2+10x+10.P(x)=2x^5+5x^4+10x^3+10x^2+10x+10.P(x)=2x5+5x4+10x3+10x2+10x+10. We need an integer aaa such that all real roots of P(x)P(x)P(x) lie in (a,a+1)(a,a+1)(a,a+1).

  2. Rewrite the polynomial in a useful form. Using (x+1)5=x5+5x4+10x3+10x2+5x+1,(x+1)^5=x^5+5x^4+10x^3+10x^2+5x+1,(x+1)5=x5+5x4+10x3+10x2+5x+1, we get 2(x+1)5=2x5+10x4+20x3+20x2+10x+2.2(x+1)^5=2x^5+10x^4+20x^3+20x^2+10x+2.2(x+1)5=2x5+10x4+20x3+20x2+10x+2. This is not exactly P(x)P(x)P(x), so instead observe: P(x)=(x+1)^5+igl(x^5+5x^3+5x^2+5x+9igr), which is not especially helpful. So let us analyze roots directly.

  3. First, determine how many real roots P(x)P(x)P(x) can have. Differentiate: P′(x)=10x4+20x3+30x2+20x+10=10(x4+2x3+3x2+2x+1).P'(x)=10x^4+20x^3+30x^2+20x+10=10(x^4+2x^3+3x^2+2x+1).P′(x)=10x4+20x3+30x2+20x+10=10(x4+2x3+3x2+2x+1). Now factor: x4+2x3+3x2+2x+1=(x2+1)(x2+2x+1)=(x2+1)(x+1)2.x^4+2x^3+3x^2+2x+1=(x^2+1)(x^2+2x+1)=(x^2+1)(x+1)^2.x4+2x3+3x2+2x+1=(x2+1)(x2+2x+1)=(x2+1)(x+1)2. Hence, P′(x)=10(x2+1)(x+1)2≥0,P'(x)=10(x^2+1)(x+1)^2\ge 0,P′(x)=10(x2+1)(x+1)2≥0, and in fact P′(x)=0P'(x)=0P′(x)=0 only at x=−1x=-1x=−1. Therefore P(x)P(x)P(x) is an increasing function on R\mathbb RR (non-decreasing everywhere, strictly increasing except for a stationary point at x=−1x=-1x=−1). So P(x)P(x)P(x) can have at most one real root.

  4. Now locate that real root by checking values. Compute: P(−2)=2(−32)+5(16)+10(−8)+10(4)+10(−2)+10P(-2)=2(-32)+5(16)+10(-8)+10(4)+10(-2)+10P(−2)=2(−32)+5(16)+10(−8)+10(4)+10(−2)+10 =−64+80−80+40−20+10=−34<0.=-64+80-80+40-20+10=-34<0.=−64+80−80+40−20+10=−34<0. Next, P(−1)=2(−1)+5(1)+10(−1)+10(1)+10(−1)+10P(-1)=2(-1)+5(1)+10(-1)+10(1)+10(-1)+10P(−1)=2(−1)+5(1)+10(−1)+10(1)+10(−1)+10 =−2+5−10+10−10+10=3>0.=-2+5-10+10-10+10=3>0.=−2+5−10+10−10+10=3>0. Since P(x)P(x)P(x) is continuous, there is a real root in (−2,−1)(-2,-1)(−2,−1).

  5. Because P(x)P(x)P(x) has at most one real root and we have found one in (−2,−1)(-2,-1)(−2,−1), this is the only real root, and hence all real roots lie in the interval (−2,−1).(-2,-1).(−2,−1). Thus, a=−2.a=-2.a=−2. Therefore, ∣a∣=2.|a|=2.∣a∣=2.

  6. Compare with the stored correct answer. Stored correct answer = 222, which matches our result.

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