jika f(2x+1)=(x-12)(x+13) , maka nilai dari f(31) adalah

f(2x+1) = x^2 + x - 156
misal: a= 2x+1
2x = a - 1
x = (a-1)/2, maka
f(a) = {(a-1)/2}^2 +(a-1)/2-156
f(a) ={(a^2-2a+1)/4}+(2a-2)/4 -156
f(a) =(a^2 -1)/4 -156
f(x) = (x^2 -1)/4 -156
f(31)= (31^2 -1)/4-1562
= (961-1)/4 - 156
= 240 -156
= 84

jika f=31.
31(2x+1)=(x-12)(x+13)
62x+31=2x+13x-12x-156
62x-2x-13x+12x=-31-156
79x=-187
x=2,36~2,40.