[tex]\frac{tg\alpha}{ctg\alpha}*(1 - sin^2\alpha) = tg\alpha*\frac{1}{tg\alpha} * cos^2\alpha = cos^2\alpha" alt="cos^2\alpha - 1 = cos^2\alpha - (sin^2\alpha + cos^2\alpha ) = - sin^2\alpha
[tex]\frac{tg\alpha}{ctg\alpha}*(1 - sin^2\alpha) = tg\alpha*\frac{1}{tg\alpha} * cos^2\alpha = cos^2\alpha" align="absmiddle" class="latex-formula">