3. Sum and Difference of Angles Formulas cos (a + 3) = cos a cos B + sin a sin B; sin (a + 3) = sin a cos B + cos a sin B. tan a + tan B tan (a + B) = 1 + tan a tan B Answer the following questions. (a) Find the exact value of cos 117 cos 87 + sin 1170 sin 870. (b) Show that the difference quotient of tan x satisfies tan(x + h) - tan x sin (h) = sec(x + h) sec x. h h