How do you simplify sin(tan^-1(7x)-sin^-1(7x))?

1 Answer
Aug 1, 2016

=(7x)/sqrt(1+49x^2)(sqrt(1-49x^2)-1), x in [-1/7, 1/7]

Explanation:

Let a = sin^(-1)(7x) in Q1 or Q4, for the principal value..

Then, sin a = 7x in [-1. 1], and so, x in[-1/7, 1/7].

and cos a =sqrt(1-49x^2) in [0, 1].

Let b = tan^(-1)(7x) in Q1 or Q4.

Then, tan b =7x in[-1, 1] and sin b =(7x)/(1+49x^2).

cos b = 1/sqrt(49x^2+1) in [0, 1].

Now, the given expressionis

sin(b-a)

=sin b cos a - cos b sin a0

=(7x)/sqrt(1+49x^2)sqrt(1-49x^2)-(1/(sqrt(1+49x^2)))(7x)

=(7x)/sqrt(1+49x^2)(sqrt(1-49x^2)-1)

Note that both sines and tangents of a and b are

>=0, when x>=0

and are <=0, when x<=0.

Cosines of of both a and b are >=0..