sin²a · ( 1 + ctg²a ) = *** ctga = cosa/sina ⇒ ctg²a = cos²a/sin²a ***
= sin²a · ( 1 + cos²a/sin²a ) = *** 1 + cos²a/sin²a = ( sin²a + cos²a )/sin²a ***
= sin²a · ( sin²a + cos²a )/sin²a = *** sin²a + cos²a = 1 ***
sin²a · 1/sin²a = *** сокращаем sin²a ***
= 1 - ответ.
