If the tangent to the curve $y=x^{3}-x^{2}+x$ at the point $(a, b)$ is also tangent to the curve $y = 5{x^2} + 2x - 25$ at the point (2, $-$1), then $|2a + 9b|$ is equal to __________.
Answer (integer)
195
Solution
Slope of tangent to curve $y=5 x^{2}+2 x-25$
<br/><br/>$=m=\left(\frac{d y}{d x}\right)_{\mathrm{at}(2,-1)}=22$
<br/><br/>$\therefore \quad$ Equation of tangent $: y+1=22(x-2)$
<br/><br/>$\therefore \quad y=22 x-45$.
<br/><br/>Slope of tangent to $y=x^{3}-x^{2}+x$ at point $(a, b)$
<br/><br/>$=3 a^{2}-2 a+1$
<br/><br/>$3 a^{2}-2 a+1=22$
<br/><br/>$3 a^{2}-2 a-21=0$
<br/><br/>$\therefore \quad a=3$ or $-\frac{7}{3}$
<br/><br/>Also $b=a^{3}-a^{2}+a$
<br/><br/>Then $(a, b)=(3,21)$ or $\left(-\frac{7}{3},-\frac{151}{9}\right)$.
<br/><br/>$\left(-\frac{7}{3},-\frac{151}{9}\right)$ does not satisfy the equation of tangent
<br/><br/>$\therefore \quad a=3, b=21$
<br/><br/>$\therefore|2 a+9 b|=195$
About this question
Subject: Mathematics · Chapter: Application of Derivatives · Topic: Tangents and Normals
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