Question

sina=3/5,sin(a+B)=5/13(a,B为第一象限角)则cosB=

Answer 1

sina=3/5

cosa=4/5

sin(a+b)=5/13

cos(a+b)=+(-)12/13

cosb=cos(a+b)*cosa-sin(a+b)*sina

=+(-)12/13*4*5-5/13*3/5

=(-15+(-)48)/65

∵b在第一象限,所以cosb大于0

所以cosb=33/65