tolong ya...........

y = a sin (bx + c)

=>
30° sampai 120° adalah setengah periode = 120° - 30° = 90°
1 periode = 180° = π

periode = 2π/b
π = 2π/b
b = 2
misal, a = 2

=>
saat x = 30°, y = 1
y = a sin ( bx + c )
1 = 2 sin ( 2.30° + c )
1 = 2 sin ( 60° + c )
misal c = -30°
1 = 2 sin 30° [terbukti]

saat 120°, y = -1
y = a sin ( bx + c )
-1 = 2 sin ( 2.120° + c )
-1 = 2 sin ( 240° + c )
misal c = -30°
-1 = 2 sin 210° [terbukti]

Jadi, a = 2, b = 2, c = - 30°

Persamaannya =
y = 2 sin ( 2x - 30° )

Semoga benar yaa