Докажите, что общее сопротивление R трёх параллельно соединенных проводников сопротивлениями R1 R2 R3 выражается формулой

Dear Tom,
Thank you for your letter.
You ask me about school uniform. I agree with you it is terrible. In my school we wear uniform too. As for changing a school, I have never changed my school. I have a lot of friends - they are my classmates. Besides, I want to tell you that I make a new friends easy. Last summer I was at a camp and I made a lot of friends there. 
By the way, have you ever been at a camp? Do you like summer camp? How do you spend your summer holidays?
Write back soon.
All the best,
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