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This is a trigonometric identity problem.
Using the identity sin(2θ) = 2sin(θ)cos(θ), we can simplify the expression:
sin(21°)cos(39°)sin(69°)cos(39°)
= (sin(221°)/2)(sin(239°)/2)
= sin(42°)sin(78°)/4
Now, using the identity sin(180°-θ) = sin(θ), we can simplify further:
sin(42°)sin(78°)/4
= sin(138°)sin(42°)/4
= (sin(180°-42°)sin(42°))/4
= sin^2(42°)/4
So the final answer is: sin^2(42°)/4.