f(x)=5x^2+1 dan g(x)=1/x +1 jika (fog)(a)=21,maka a=.... a.-1/3 b.0 c.1 d.-1/3 atau 1 e.0 atau 1

jawab

fog(x) =  f {g(x)}  = f {1/x  +1 }
f og (x) =  5 (1/x  + 1)^2 + 1
f og(a) = 21

5 ( 1/a + 1)^2 + 1 = 21
5 (1/a  + 1)^2 = 20
(1/a  + 1)^2 = 4
1/a + 1 = 2  atau 1/a + 1 = - 2
1/a = 1  atau 1/a = - 3
a = 1  atau a = - 1/3
jawab D