using cosine rule for these 2 forces at an angle x ,c is resultant, a and b are the 2 other forces,
c = sqrt(a^2+b^2-2abcosx)
a. becomes maximum when cosx =0 or x=90degrees.
c = sqrt(100^2 + 40^2 -2x100x40Cos90)
c = 107.7 N as resultant
b. becomes minimum when cosx =1 or x=0 degrees.
c =sqrt(100^2 + 40^2 -2x100x40Cos0)
c = 60 N as resultant
c. Range of forces will be from 60 N up to 107.7 N
d. It can not, because it is not within the range.
= sqrt(100^2 + 40^2 -2x100x40Cos0
= 60 N