tan(2a-b)=tan[a+(a-b)]=[tana+tan(a-b)]/[1-tanatan(a-b)]
=(1/2-2/5)/(1+1/2*2/5)
=(1/10)/(6/5)
=1/12
tan(b-2a)=-tan(2a-b)=-1/12
tan(2a-b)=tan[a+(a-b)]=[tana+tan(a-b)]/[1-tanatan(a-b)]
=(1/2-2/5)/(1+1/2*2/5)
=(1/10)/(6/5)
=1/12
tan(b-2a)=-tan(2a-b)=-1/12