A.Maths Trigonometry

9sinΦ - 40cosΦ = √(92 + 402) (9 sin Φ /√(92 + 402) - 40 cos Φ/√(92 + 402))
= 41 (9 sin Φ /41 - 40 cos Φ/41)
= 41 (sin 12.7 sin Φ - cos 12.7 cos Φ)
= 41 cos (Φ + 12.7)

2008-08-03 22:37:13 補充：
Sorry, redo:
9sinΦ- 40cosΦ=41 (9 sin Φ /41 - 40 cos Φ/41)
= 41 (sin 167.3 sin Φ + cos 167.3 cos Φ)
= 41 cos (Φ - 167.3)

Please refer to the following solution:

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