sin 20 cos20 = -sin50
sin 70 cos70
Please specify what do you want for your question
If you want to prove sin20 cos20 = sin70 cos70
Then sin20cos20 = 1/2 ( sin40 ) [ as 2sinxcosx = sin2x]
And 1/2(sin180 - 40 ) = 1/2 (sin40 ) [ as sin (180 -x ) = sinx]
So 1/2sin40 = 1/2 sin140 = 2/2 (sin70cos70) = sin70cos70
If you want to prove sin20 cos20 = sin70 cos70
Then sin20cos20 = 1/2 ( sin40 ) [ as 2sinxcosx = sin2x]
And 1/2(sin180 - 40 ) = 1/2 (sin40 ) [ as sin (180 -x ) = sinx]
So 1/2sin40 = 1/2 sin140 = 2/2 (sin70cos70) = sin70cos70