# Consider the function f(x) = sin-1(3x-4x3) + cos-1(4x3-3x)Let the dreivative of the function be, f'(x) = D1+D2where D1 = (d/dx)(sin-1(3x-4x3))   -------------(1)and D2 = (d/dx)(cos-1(4x3-3x))    ------------(2)Let x = sinθ so that (1) becomes;D1 = (d/dx)(sin-1(3sinθ-4sin3θ))ie, D1 = (d/dx)(sin-1(sin 3θ)ie, D1 = (d/dx)(3θ)ie, D1 = (d/dx) (3 sin-1 x)similarly, putting x = cos φ in (2), we can show thatD2 = (d/dx) (3 cos-1 x)Therefore, f'(x) = D1+D2= (d/dx) (3 sin-1 x) + (d/dx) (3 cos-1 x)= (d/dx)(3(sin-1 x+ cos-1 x)= (d/dx)(3π÷2)=0But, We can prove that the aboce given function f(x) is not a constant function either by substituting 3x-4x3 = y or using a scientific calculator!!!How this is possible? Can the derivative of a non-constant function be zero???

krishna sandilya
8 Points
14 years ago
dude, there is no rule that the derivative of a function is not zero it is always zero for a constant function one thing is that you have definitely gone wrong when you have substituted 3theta for arcsin[sin[theta]] by the way may i know the values of your scientific calculator that has shown dif values
148 Points
14 years ago

Dear Davis Devasia

sin-1 x+ cos-1 x =∏/2

you can use this formula if x is same in both sin-1x and cos-1x

but here you can use it because x in sin-1x is sinΘ

and x in cos-1x is cosΦ   ,both are different .you can use this formula

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