p=cos²a+(cosacos60+sin60sina)²+(cosacos60-sin60sina)²
=cos²a+cos²acos²60+2cosacos60sin60sina+sin²60sin²a+cos²acos²60-2cosacos60sin60sina+sin²60sin²a
=cos²a+2cos²acos²60+2sin²60sin²a
=cos²a+2cos²a*1/4+2*3/4sin²a
=cos²a+2cos²a*1/4+2*3/4sin²a
=3/2*cos²a+3/2*sin²a
=3/2*(cos²a+sin²a)
=3/2