相關變化率 - 經濟

題目:
A country exports electronic components E and textiles T.
Suppose at a particular time t=t0 the revenue derived from
each of these goods is E(t0)=11 and T(t0)=8
and that E is growing at 9%, while T is declining at 2%.
At what relative rate is total export revenue R=E+T changing at this time.

我的想法:
一般相關變化率應該都會給dx/dt或dy/dt
這一題 都沒給 怎麼算?
或是給了 但我沒看懂?
請先進指教一下

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