calculati:
a)(1/2-3/4)-(5/2-5/4)-(3/2-3/5)
b)(-2/3+4/5+1)-(-4/3+2/5-2)+(-2+3/5)
c)(1-3/4+2/3)-(1-5/3+1/4)-(1-4/3-3/4)
d)1/2-(1/3+1/4-1/5)