Diketahui segitiiga PQR , PQ=2 QR=4 ,& sudut Q=120 maka cos P =......
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Digunakan:
PR² = PQ²+QR²-2.PQ.QR.cos Q
PR² = 2²+4²-2.2.4.cos 120
PR² = 4+16-16.(-1/2)
PR² = 20 + 8
PR² = 28
PR = √28 = 2√7
Sehingga, dengan aturan cosinus:
cos P
= (PQ²+PR²-QR²)/(2.PQ.PR)
= (2²+28-4²)/(2.2.2√7)
= (4+28-16)/8√7
= 16/8√7
= 2/√7
= 2/7 √7 (Rasionalisasi)