Differentiate
y = (r² – 9r) · eʳ
with respect to r.
As y is a product of two functions, then use the product rule:
dy d
—— = —— [ (r² – 9r) · eʳ ]
dr dr
dy d d
—— = —— (r² – 9r) · eʳ + (r² – 9r) · —— (eʳ)
dr dr dr
• The derivative of r² – 9r is 2r – 9;
• The derivative of eʳ is eʳ.
Therefore,
dy
—— = (2r – 9) · eʳ + (r² – 9r) · eʳ
dr
dy
—— = [ (2r – 9) + (r² – 9r) ] · eʳ
dr
dy
—— = (r² – 7r – 9) · eʳ <——— this is the answer.
dr
I hope this helps. =)
