【佟硕公益数学】-三角函数知识点总复习+高考真题选讲

P5三角恒等变换（下)例7

cos[(π/4-a)-(3π/4+β)]

=cos(-π/2-a-β)

=cos(π/2+a+β)

=sin(a+β)

sin(a+β)=(3/5)(12/13)+(5/13)(4/5)

=56/65