Question 286846
write the first three terms of the expansion (a-b)^6 
--------------------
Use Pascal's triangle
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1  6th power
---------------------------
Notice that each element in the triangle is the sum of 2 above it, so you can fill it in quickly for "small" exponents.  I wouldn't do it for an exponent of 50.
Since it's a-b, the signs alternate.
--> {{{a^6 - 6a^5b + 15a^4b^2 - 20a^3b^3}}}
The exponents of a decrease while b's increase.