From Pythagorean Identities <span><span><span>sin^2</span>x</span>+<span><span>cos^2</span>x</span>=1</span>
and from that identities we can have <span><span>sinx</span>+<span>cosx</span>=1</span>, taking <span><span>sinx</span>=1−<span>cosx</span></span> and substitute in the equation <span><span>sinx</span>+<span>cosx</span>=1</span> <span><span>(1−<span>cosx</span>)</span>+<span>cosx</span>=1</span>
Remove the parenthesis <span>1−<span>cosx</span>+<span>cosx</span>=1</span>
cancel <span>cosx</span> <span>1=1</span>