Find the temperature of a tungsten filament

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Find the temperature of the filament of a tungsten incandescent lamp in working condition, if it is known that the resistance of the filament at the time of switching on at a temperature t1 = 20 ° C is 12.6 times less than in the working state.
Task 30989. A detailed solution with a brief record of the conditions, formulas and laws used in the solution, the derivation of the calculation formula and the answer.
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