Given: f′(x)>2f(x) ∀x∈R
f(0)=1
To find: the correct statement among (1)–(4)
Let g(x)=f(x)e−2x
g′(x)=e−2x(f′(x)−2f(x))>0
g strictly increasing,g(0)=f(0)e0=1
x>0⇒g(x)>g(0)=1
⇒f(x)e−2x>1⇒f(x)>e2x
⇒(3) TRUE
f(x)>e2x>0⇒f′(x)>2f(x)>0⇒(1) TRUE
f′(x)>2f(x)>2e2x>e2x⇒(4) false
f increasing⇒(2) false
TRUE: (3) f(x)>e2x, and (1) f increasing
(1) and (3)