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

2x + 1 = 31
2x = 30 - 1
2x= 30
x = 15

f(31) = (15 -12)(15 + 13)
= 3 x 28
= 84

y = 2x+1
2x = y-1
x = y-1 / 2
x~ = x-1 / 2


f( 2x +1) = (x-12)(x+13)
f(x) = (x~- 12)(x~ +13)
= (x-1 /2 - 12)(x~ + 13)
f(31) = (31-1 /2 - 12)(31-1 /2 + 13)
= (30/2 - 12)(30/2 + 13)
= (15-12)(15+13)
= (3)(28)
= 84

atau
2x+1 = 31
2x = 30
x = 15

f(31) = (15-12)(15+13)
= (3)(28)
= 84