Length (6) strings (4) symbols से बनती हैं और हर symbol कम से कम एक बार आए। Count कौन-सी है?
Length (6) strings are formed from (4) symbols and every symbol appears at least once. Which count is correct?
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A. (\sum_{i=0}^{4}(-1)^i{}^{4}C_i(4-i)6)
Simple Explanation
हर symbol का आना onto condition है, इसलिए missing symbols को inclusion-exclusion से हटाते हैं। परीक्षा में at least once को onto mapping समझें। / Every symbol appearing is an onto condition, so missing symbols are removed by inclusion-exclusion. In exams treat at least once as onto mapping.
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