How do I find the value of tan (3Pi/4)?

##-1##
Basically when they say ##pi## they mean ##pi## radians.
##pi ## radians = ##180^@##
So
##tan((3pi)/4)##
= ##tan((3xx180^@)/4)##
= ##tan((3xx45cancel(180)^@)/cancel(4))##
= ##tan(3xx45^@)##
= ##tan(135^@)##
= ##-1##
Note:
The CAST rule says that tan should be negative in the second quadrant ##(90^@ – 180^@)##
Basic values of tan are
tan0 = 0
tan 30 = ##1/sqrt(3)##
tan45 = 1
tan60 = ##sqrt(3)##
tan90 = undefined (or ##oo##)
You can add and subtract angles to and from 180 and 360 to get the same angle provided you follow the cast rule for the sign.
So
##tan(180-x) = – tanx##
likewise
##tan135=tan(180-45)=-tan45=-1##