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a / sin A = b / sin B
4 / sin A = 6 / sin 45°
sin A = [sin 45° x 4] / 6
sin A = [√2/2 x 4] / 6
sin A = √2/3
Buat segitiga siku-siku
⇒ Panjang sisi √2 di depan sudut A
⇒ Sisi miringnya 3
⇒ Sisi samping = √ [3² - (√2)²] = √7
∴ cosinus A = √7/3. (E)