How to find a vector with a polar angle and magnitude

find the vector v=ai+bj which has a polar angle of 5pi/4 and a magnitude of 5.

5pi/4 = (pi+pi/4 )
The vector is v=5cospi/4i 5sinpi/4 j

But 5pi/4 = (pi+pi/4 ) , so
cos 5pi/4 = -cos pi/4 = -(1/2) sqrt2
sin 5pi/4 = -sin pi/4 =-(1/2) sqrt2

v= (-5/2) sqrt2 ( i+j)