Dla pewnego kąta ostrego spełniony jest warunek sin + cos= oblicz sin * cos
( sin alfa + cos alfa )^2 = sin^2 alfa + 2 sin alfa * cos alfa + cos^2 alfa =
= 1 + 2 sin alfa * cos alfa
więc
2 sin alfa *cos alfa = ( sin alfa + cos alfa)^2 - 1 / : 2
sin alfa * cos alfa = 0,5 *[ sin alfa + cos alfa ]^2 - 1/2
Mamy zatem
sin alfa* cos alfa = 0,5*[ 3 p(5)/5 ]^2 - 1/2 = 0,5 *[ 9* ( 5/25)] - 1/2 =
= 0,5 * ( 9/5) - 0,5 = 9/10 - 5/10 = 4/10 = 2/5
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( sin alfa + cos alfa )^2 = sin^2 alfa + 2 sin alfa * cos alfa + cos^2 alfa =
= 1 + 2 sin alfa * cos alfa
więc
2 sin alfa *cos alfa = ( sin alfa + cos alfa)^2 - 1 / : 2
sin alfa * cos alfa = 0,5 *[ sin alfa + cos alfa ]^2 - 1/2
Mamy zatem
sin alfa* cos alfa = 0,5*[ 3 p(5)/5 ]^2 - 1/2 = 0,5 *[ 9* ( 5/25)] - 1/2 =
= 0,5 * ( 9/5) - 0,5 = 9/10 - 5/10 = 4/10 = 2/5
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