Find the value ofaand the value ofbso that the following is a valid c.d.f.

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So far I did,

F(3) = 1/b (2x^3 + ax – 2a) = 1

=> 1/b(54+a)=1 ------------------(1)

F(1) = 1/b (2x^3 + ax – 2a) = 0

=> 1/b (2-a) = 0 -----------------(2)

From (1) & (2):

a = 2 & b = 56.

My book’s answers says: a = 2 & b =1/ 56.

Is this method right? Where I did worng?