Question 617165
{{{cos^2(theta)-cos(2theta)}}}
There are three variations of the {{{cos(2theta)}}} formula:<ul><li>{{{cos(2theta) = cos^2(theta)-sin^2(theta)}}}</li><li>{{{cos(2theta) = 2cos^2(theta)-1}}}</li><li>{{{cos(2theta) = 1-2sin^2(theta)}}}</li></ul>Any of them can be used. I'm choosing the first one because I see the rest being slightly faster/easier by choosing it:
{{{cos^2(theta)-(cos^2(theta)-sin^2(theta))}}}
Note the use of parentheses! It is very important to use them when substituting in a multiple-term expression. Here it helps us see that the {{{sin^2(theta)}}} is supposed to get subtracted, too. The {{{cos^2(theta)}}}'s cancel and the subtraction of {{{-sin^2(theta)}}} leaves us with:
{{{sin^2(theta)}}}