Chứng minh rằnga - a - b + c + d - d + c - b - 2a

Chứng minh rằng:

a) (left( { - a - b + c + d} right) - left( {d + c - b - 2a} right) = a)            b) (left( {m + n} right) - left[ {p + left( {n + m - p} right)} right] = 0)

c) (left( {x - 35} right) - left[ {left( {15 + x} right) - left( {48 + x} right)} right] = x - 2)    d) (a + left{ {left( {a - 3} right) - left[ {left( {a + 3} right) - left( { - a - 2} right)} right]} right} =  - 8)