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i needed that answer BEFORE school started not 2 HOURS IN! I appreciate you FINALLY ANSWERING but how bout a little bit better timing. I don't think i will be coming back.

Quote:By letting x+y=u and x-y=v in sinxsiny=1/2[cos(x-y)-cos(x+y)], show that:

cosv-cosu=2sin((u+v)/2)sin((u-v)/2)

We have sinxsiny=1/2[cos(x-y)-cos(x+y)],

Taking 2 to other side

2sinxsiny=[cos(x-y)-cos(x+y)] .............2

Now put
x+y=u
x-y = v

Adding above two equations we have

x=(u+v)/2
y=(u-v)/2

Put all thes values in the equation .............2

2 sin(u+v)/2 sin (u-v)/2 = cosv-cosu
Hence proved

if you can help me with that last one! that is all i need to do and i am done. Please help!

By letting x+y=u and x-y=v in sinxsiny=1/2[cos(x-y)-cos(x+y)], show that:

cosv-cosu=2sin((u+v)/2)sin((u-v)/2)

sinx
-------- = (csc x)(1+cos x)
1-cosx

sinx
-------- = (1/sinx)(1+cos x)
1-cosx

Cross multiply
Sin^2 x = (1-Cos x)(1+Cosx)

Sin^2x = 1-Cos^2 x

Sin^ x +Cos ^2 x = 1 Which is standard identity