Question 204705
there is an identity formula of trigonometry :
{{{sin^2 (x) + cos^2 (x) = 1}}}


by using that formula: 
{{{cos^2 (x) = 1 - sin^2 (x)}}}


so:
{{{sin^2 (x) - cos^2 (x) = sin^2 (x) - (1 - sin^2 (x))}}}
= {{{sin^2 (x) - 1 + sin^2 (x)}}}
= {{{2sin^2 (x) - 1}}}