A Maths問題

a) 9sinx-40cosx=rsinxcosy+rcosxsiny
rcosy=9....i
rsiny=-40.....ii
i2+ii2
r2(cos2y+sin2y)=92+402
r2=1681
r=41
ii/i
tany=-40/9
y=-77.32
9sinx-40cosy=41sin(x+77.32)
-1<= sin(x+77.32) <= 1
-41 <= 41sin(x+77.32) <= 41

b 同 c 睇唔清D符號 = =""

2008-05-13 21:39:39 補充：
其實都係用 subsidiary angle 去計

asinx+bcosx = rsin(x+y) = rsinxcosy-rcosxsina

rcosy=a
rsinx=b

r^2 = (a^2 +b^2)
tany = b/a

用呢個方法就可以令 sin 同 cos 變成得翻 sin 或者cos
跟住就可以輕鬆咁計出佢個max 同 min