# A simple problem in two dimensional optimization

I have the following problem: given $$f_1=\frac{-12d_1}{\pi d_2-12}$$ and: $$f_2=\frac{\pi d_1d_2}{\pi(d_1+d_2)-12}$$ I need to find the maximum of the following function: $$F(d_1,d_2)=|f_1|+|f_2|$$ with the following constrains: $$|f_1|\ge a$$ $$|f_2|\ge b$$ Thanks in advance.

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