Question

can triangle ABC be written as - BCA,ACB or CAB ?Give reasons

Answer 1

Yes, a Triangle can be represented in any of the above mentioned ways but each side's name (A, B & C) should be mentioned; not necessarily in a particular order.
So, 
ΔABC = ΔBCA = ΔACB = ΔCAB
All denote the same thing.

Answer 2

Yes. While writing the name order doesn't matter. There are 6 ways of writing a triangle's name. eg ABC, ACB, BCA, BAC, CAB & CBA.