From the construction of the expression you can see you shou

From the construction of the expression, you can see you should use the for. Write the expression as the cosine of a sum. Then simplify to write the expression as a trigonometry of one number. cos(19 pi/20) cos (-pi/5) - sin(19 pi/20) sin(- pi/5) = cos(+ (- pi/5)) = cos() =

Solution

the answer should be right. may be it is showing wrong if it has asked the decimal value which is numerically equal to 0.707 .