Question

Solve cos315*cos(-390)-sin225*sin750

cos315:
<RA = 360-315
= 45
cos45 = 1/√2

cos-390:
<RA=-390+360
=30
cos30= √3/2

sin225:
<RA = 225-180
= 45
Sin45=1/√2

Sin750:
<RA = 750 - 2(360)
=30
Sin30=1/2

Put it all together...
(1/√2)(√3/2)-(1/√2)(1/2)
=√3/2√2 - 1/2√2

The answer in the back is √2 + √6/4...

What did I do wrong?

Answer 1

sin225 is in the third quadrant, sine is negative.

cos-390 is in the fourth quadrant, positive.

change that sign, you get it (after rationalizing the denominator)

Answer 2

Thanks^