Jika sin 2a + sin 2b = 1/2 dan cos 2a + cos2b = 3/2, maka cos^2(ab) sama dengan (A) 3/8(B) 5/8 (C) (D) 5/ 4

sinc+sind=2 [sin(c+d)/2] [cos (cd)/2]

sin2a+sin2b=2sin(a+b)*cos(ab)

2sin(a+b) * cos(ab) = 1/2 —–(1)

cos2a+cos2b=3/2

cosc+cosd=2 [cos(c+d)/2] [cos (cd)/2]

cos2a+cos2b=2cos(a+b)*cos(ab)

2cos(a+b) * cos(ab) = 3/2 —–(2)

mengkuadratkan dan menjumlahkan (1) dan (2) kita peroleh,

4sin^2(a+b) * cos^2(ab) + 4cos^2(a+b) * cos^2(ab) = 9/4+1/4

4 cos^2(ab) [sin^2(a+b)+cos^2(a+b)] = 10/4

cos^2(ab) = 10/16

cos^2(ab) = 5/8

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