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Well these areas are genetically darker than rest of the body.But there are certain creams which you can use like hydroquinone
cream, glycolic creams ,salicylic acid
creams ,tretenoin creams etc. Combination of such creams can give you good result ,but then you have to maintain it too.You should also use a good spf sunscreen (medicated) at least three times a day.
At times if the area is very resistant to creams then I give some off labeled injections too,which have given good results in my patients.
You can even try taking lasers ,like Nd;yag laser ,which works best for pigmentation.
If you can let me know where you belong to ,then probably I can suggest you a good laser clinic near by.