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Cos 20.cos100 plus cos100.cos140 minus cos140.cos200 is equal to
cosA + cosB = 2[cos(A + B)/2][cos(A - B)/2] i.e. (1/2)(cosA + cosB) = [cos(A + B)/2][cos(A - B)/2] => cos20cos100 = (1/2)(cos120 + cos80) => cos100cos140 = (1/2)(cos240 + cos40) => -cos140cos200 = -(1/2)(cos340 + cos60) => (1/2)[cos120 + cos80 + cos240 + cos40 – cos340 – cos60] => (1/2)[-1/2 + cos80 – 1/2 + cos40 – cos340 – 1/2] => (1/2)[-3/2 + cos80 + cos40 – cos340] => (1/2)[-3/2 + 2cos60cos20 – cos340] => (1/2)[-3/2 + cos20 – cos340] => (1/2)[-3/2 + cos20 – cos(-20)] So, (1/2)(-3/2) = -3/4
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