Show d/dx (cos^-1 2x)=-2/(1-4x^2)^(1/2). If y=sin (cos^-1 2x),show that (1-4x^2) (d^2y)/(dx^2)-4x dy/dx+4y=0. How to do this?

2 Answers
Apr 28, 2018

We want to show that:

d/dx cos^(-1)(2x)=-2/(1-4x^2)^(1/2)
y =sin( cos^(-1)(2x)) => (1-4x^2) (d^2y)/(dx^2)-4x dy/dx+4y=0

We start with the standard calculus result:

d/dx cos^(-1)x = -1/sqrt(1-x^2)

Then applying the chain rule, we have:

d/dx cos^(-1)(2x) = -1/sqrt(1-(2x)^2) d/dx (2x)
" " = -2/sqrt(1-4x^2) \ \ \ QED

so then, given y =sin( cos^(-1)(2x)), then applying the chain rule, we have:

dy/dx = cos( cos^(-1)(2x)) d/dx cos^(-1)(2x)
\ \ \ \ \ = (2x) (-2/sqrt(1-4x^2))
\ \ \ \ \ = (-4x) / sqrt(1-4x^2)

And then by applying the quotient rule:

(d^2y)/(dx^2) = { (sqrt(1-4x^2))(d/dx -4x) - (-4x)(d/dx sqrt(1-4x^2)) } / (sqrt(1-4x^2))^2

\ \ \ \ \ \ = { (sqrt(1-4x^2))(-4) + 4x(1/2(1-4x^2)^(-1/2)d/dx(-4x^2)) } / (1-4x^2)

\ \ \ \ \ \ = { (sqrt(1-4x^2))(-4) + 4x(1/2(1-4x^2)^(-1/2)(-8x)) } / (1-4x^2)

\ \ \ \ \ \ = { -4sqrt(1-4x^2) - (16x^2)/sqrt(1-4x^2) } / (1-4x^2)

And so the LHS of the given DE is:

LHS = (1-4x^2) (d^2y)/(dx^2)-4x dy/dx+4y

\ \ \ \ \ \ \ \ = (1-4x^2) {{ -4sqrt(1-4x^2) - (16x^2)/sqrt(1-4x^2) } / (1-4x^2)} -4x {(-4x) / sqrt(1-4x^2)} +4sin( cos^(-1)(2x))

\ \ \ \ \ \ \ \ = -4sqrt(1-4x^2) - (16x^2)/sqrt(1-4x^2) +(16x^2) / sqrt(1-4x^2) +4sin( cos^(-1)(2x))

\ \ \ \ \ \ \ \ = 4sin( cos^(-1)(2x)) -4sqrt(1-4x^2)

Now if we use the trigonometric identity sin^2 A + cos^2A -=1, then:

(sin (cos^(-1)2x))^2 + (cos(cos^(-1)2x))^2 -=1

:. (sin (cos^(-1)2x))^2 =1 - (cos(cos^(-1)2x))^2
:. sin (cos^(-1)2x) =sqrt(1 - (2x)^2) = sqrt(1 - 4x^2)

So, we have:

LHS = 4{sqrt(1 - 4x^2)} -4sqrt(1-4x^2)
\ \ \ \ \ \ \ \ = 0 \ \ \ QED

Apr 28, 2018

Please refer to the Explanation.

Explanation:

We differentiate using the Chain Rule :

d/dx(cos^-1 2x),

=-1/sqrt{1-(2x)^2}*d/dx(2x),

=-1/sqrt(1-4x^2)*(2).

rArr d/dx(cos^-1 2x)=-2/(1-4x^2)^(1/2), as desired!

For ease of writing, we will use the following notations :

dy/dx=y_1 and (d^2y)/dx^2=d/dx(dy/dx)=d/dx(y_1)=y_2.

Note that : (1) : d/dx(y^2)=2y*dy/dx=2yy_1.

(2) : d/dx(y_1^2)=2y_1*d/dx(y_1)=2y_1y_2.

Next, we have, y=sin(cos^-1 2x).........(star_1).

:. dy/dx=cos(cos^-1 2x)*d/dx(cos^-1 2x).

:. y_1=cos(cos^-1 2x)*{-2/sqrt(1-4x^2)}..."[from above]".

:. -1/2sqrt(1-4x^2)y_1=cos(cos^-1 2x)......(star_2).

Hence, squaring and adding (star_1) and (star_2), we get,

y^2+1/4(1-4x^2)y_1^2=1, or,

4y^2+(1-4x^2)y_1^2=4.

Rediff.ing w.r.t. x and using (1) & (2) above, we get,

4(2yy_1)+{(1-4x^2)(2y_1y_2)+y_1^2(-8x)}=0.

Dividing throughout by 2y_1!=0 & rearranging, we get,

(1-4x^2)y_2-4xy_1+4y=0, as desired!

Enjoy Maths!