What is the equation of the line that is normal to #f(x)=-3x^2sinx # at # x=pi/3#?

1 Answer
Feb 18, 2018

#(y-(pi)^2/6)/(x-pi/3)=(18sqrt3)/(pi(18+sqrt3)#

Explanation:

#"Let "y=-3x^2sinx#

#"Let"u=x^2, v=sinx#

#y=-uv#

#"Differentiating wrt x on both sides"#

#(dy)/dx=d/dx(y)=-d/dx(uv)#

#"By product rule", d/dx(uv)=u(dv)/dx+v(du)/dx#

#(du)/dx=2x, (dv)/dx=cosx#

#d/dx(uv)=x^2cosx+(sinx)(2x)#

#d/dx(uv)=x^2cosx+2xsinx#

#(dy)/dx=-(x^2cosx+2xsinx)#

#x=pi/3#

#y=-3x^2sinx=y=-3(pi/3)^2sin(pi/3)#

#y=-3(pi)^2/9xx1/2#

#y=-(pi)^2/6#

#P-=(x,y)=(pi/3,-(pi)^2/6)#

#dy/dx=-(x^2cosx+2xsinx)#
#=-((pi/3)^2cos(pi/3)+2(pi/3)sin(pi/3))#

#=-(pi^2/9xx1/2+(2pi)/3xxsqrt3/2)#

#dy/dx=-((pi)^2/18+pi/sqrt3)#

#"Slope of the tangent at "P-=(pi/3,-(pi)^2/6) "is"#

#m=dy/dx=-((pi)^2/18+pi/sqrt3)#

#"Slope of the normal m' is given by " m'=-1/m #

#m'=1/((pi)^2/18+pi/sqrt3)=1/((sqrt3(pi)^2+18pi)/(18sqrt3))#

#m'=(18sqrt3)/(sqrt3(pi)^2+18pi)=(18sqrt3)/(pi(sqrt3+18)#

#m'=(18sqrt3)/(pi(18+sqrt3)#

#"Equation of the normal passing through the point ", P-=(pi/3,-(pi)^2/6) "and having the slope ", m'=(18sqrt3)/(pi(18+sqrt3) " #

#"is given by"#

#(y-(pi)^2/6)/(x-pi/3)=(18sqrt3)/(pi(18+sqrt3)#