Question
Write the value of $\sum\limits_\text{r-1}^6 {^\text{56-r}}\text{C}_{\text{3}}+{^\text{50}}\text{C}_{\text{4}}.$

Answer

$\sum\limits_\text{r-1}^6 {^\text{56-r}}\text{C}_{\text{3}}+{^\text{50}}\text{C}_{\text{4}}$ $={^\text{56-1}}\text{C}_{\text{3}}+{^\text{56-2}}\text{C}_{\text{3}}+​​​​{^\text{56-3}}\text{C}_{\text{3}}+​​​​{^\text{56-4}}\text{C}_{\text{4}}+{^\text{56-5}}\text{C}_{\text{5}}+{^\text{56-6}}\text{C}_{\text{6}}+{^\text{50}}\text{C}_{\text{4}}$ $={^\text{55}}\text{C}_{\text{3}}+{^\text{54}}\text{C}_{\text{3}}+​​​​{^\text{53}}\text{C}_{\text{3}}+​​​​{^\text{52}}\text{C}_{\text{3}}+{^\text{51}}\text{C}_{\text{3}}+{^\text{50}}\text{C}_{\text{3}}+{^\text{50}}\text{C}_{\text{4}}$ $={^\text{55}}\text{C}_{\text{3}}+{^\text{54}}\text{C}_{\text{3}}+​​​​{^\text{53}}\text{C}_{\text{3}}+​​​​{^\text{52}}\text{C}_{\text{3}}+{^\text{51}}\text{C}_{\text{3}}+({^\text{50}}\text{C}_{\text{3}}+{^\text{50}}\text{C}_{\text{4}})$ $={^\text{55}}\text{C}_{\text{3}}+{^\text{54}}\text{C}_{\text{3}}+​​​​{^\text{53}}\text{C}_{\text{3}}+​​​​{^\text{52}}\text{C}_{\text{3}}+({^\text{51}}\text{C}_{\text{3}}+{^\text{51}}\text{C}_{\text{4}})$ $={^\text{55}}\text{C}_{\text{3}}+{^\text{54}}\text{C}_{\text{3}}+({^\text{53}}\text{C}_{\text{3}}+{^\text{54}}\text{C}_{\text{4}})$ $={^\text{55}}\text{C}_{\text{3}}+({^\text{54}}\text{C}_{\text{3}}+​​​​{^\text{54}}\text{C}_{\text{4}})$ $=({^\text{55}}\text{C}_{\text{3}}+{^\text{55}}\text{C}_{\text{4}})$ $={^\text{56}}\text{C}_{\text{4}}$

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