Given a = 84, find the value of the algebraic formula A-4 / A ^ 2-9 / 1 / A-3 * a ^ 2 + 2a-3 / A ^ 2-8a + 16 It's better to have a process, if not, give the answer directly

Given a = 84, find the value of the algebraic formula A-4 / A ^ 2-9 / 1 / A-3 * a ^ 2 + 2a-3 / A ^ 2-8a + 16 It's better to have a process, if not, give the answer directly

The title is too messy. I can only answer it according to my understanding. If you understand it wrong, please tell me the title again
(a-4)/(a^2-9)/1/[(a-3)(a^2+2a-3)]/(a^2-8a+16)
=(a-4)(a-3)(a^2+2a-3)/[(a^2-9)(a^2-8a+16)]
=(a-4)(a-3)(a+3)(a-1)/[(a-3)(a+3)(a-4)^2]
=(a-1)/(a-4)
=83/80

Let's show that the value of the algebraic formula 16 + a - {8A - [a-9-3 (1-2a]} has nothing to do with a,

Algebraic formula 16 + a - {8A - [a-9-3 (1-2a]}
=Algebraic formula 16 + a - {8A - [a-9-3 + 6A]}
=Algebraic formula 16 + a - {8A - [7a-12]}
=Algebraic formula 16 + a - {a + 12}
=16+A-A-12
=18
Therefore, the value of the algebraic formula 16 + a - {8A - [a-9-3 (1-2a]} has nothing to do with a

The law of 1 2 4 7 is expressed by algebraic formula

[(n-1) n] / / 2 + 1 (n is a natural number)