Question Number 2240 by Rasheed Soomro last updated on 10/Nov/15
$$\mathcal{GENERALIZE}: \\ $$$$\left({a}+{b}\right)\left({a}^{\mathrm{2}} +{b}^{\mathrm{2}} −{ab}\right)={a}^{\mathrm{3}} +{b}^{\mathrm{3}} \\ $$$$\left({a}+{b}+{c}\right)\left({a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} −{ab}−{bc}−{ca}\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:={a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} −\mathrm{3}{abc} \\ $$$$\left({a}+{b}+{c}+{d}\right)\left({a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} +{d}^{\mathrm{2}} −…..\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:={a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} +{d}^{\mathrm{3}} \:…… \\ $$$$−−−−−−−−−−−−−−−−−−−−−−−−− \\ $$$$\left({a}_{\mathrm{1}} +{a}_{\mathrm{2}} +…+{a}_{{n}} \right)\left({a}_{\mathrm{1}} ^{\:\mathrm{2}} +{a}_{\mathrm{2}} ^{\:\mathrm{2}} +…+{a}_{{n}} ^{\:\mathrm{2}} −…..\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:={a}_{\mathrm{1}} ^{\:\mathrm{3}} +{a}_{\mathrm{2}} ^{\:\mathrm{3}} +…+{a}_{{n}} ^{\:\mathrm{3}} \:….. \\ $$
Answered by Rasheed Soomro last updated on 15/Nov/15
$$\left({a}+{b}+{c}+{d}\right)\left({a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} +{d}^{\mathrm{2}} −{ab}−{ac}−{ad}−{bc}−{bd}−{cd}\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:={a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} +{d}^{\mathrm{3}} −\mathrm{3}{abc}−\mathrm{3}{abd}−\mathrm{3}{acd}−\mathrm{3}{bcd} \\ $$$$−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− \\ $$$$\left({a}+{b}+{c}+{d}\right)\left({a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} +{d}^{\mathrm{2}} −{ab}−{ac}−{ad}−{bc}−{bd}−{cd}\right) \\ $$$$\:\:\:={a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} +{d}^{\mathrm{3}} −\mathrm{3}{abc}−\mathrm{3}{abd}−\mathrm{3}{acd}−\mathrm{3}{bcd} \\ $$$$\frac{−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−}{} \\ $$$$\left(\underset{\left(\mathrm{1}\right)} {{a}+{b}}\right)\left(\underset{\left(\mathrm{2}\right)} {{a}^{\mathrm{2}} +{b}^{\mathrm{2}} }\underset{\left(\mathrm{3}\right)} {−{ab}}\right)=\underset{\left(\mathrm{4}\right)} {{a}^{\mathrm{3}} +{b}^{\mathrm{3}} }\underset{\left(\mathrm{5}\right)} {−−−−} \\ $$$$\left(\underset{\left(\mathrm{1}\right)} {{a}+{b}+{c}}\right)\left(\underset{\left(\mathrm{2}\right)} {{a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} }\underset{\left(\mathrm{3}\right)} {−{ab}−{ac}−{bc}}\right)=\underset{\left(\mathrm{4}\right)} {{a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} }\underset{\left(\mathrm{5}\right)} {−\mathrm{3}{abc}} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:−−− \\ $$$$\left(\underset{\left(\mathrm{1}\right)} {{a}+{b}+{c}+{d}}\right)\left(\underset{\left(\mathrm{2}\right)} {{a}^{\mathrm{2}} +{b}^{\mathrm{2}} +{c}^{\mathrm{2}} +{d}^{\mathrm{2}} }\underset{\left(\mathrm{3}\right)} {−{ab}−{ac}−{ad}−{bc}−{bd}−{cd}}\right) \\ $$$$\:\:\:\:\:\:\:\:\:=\underset{\left(\mathrm{4}\right)} {{a}^{\mathrm{3}} +{b}^{\mathrm{3}} +{c}^{\mathrm{3}} +{d}^{\mathrm{3}} }\underset{\left(\mathrm{5}\right)} {−\mathrm{3}{abc}−\mathrm{3}{abd}−\mathrm{3}{acd}−\mathrm{3}{bcd}} \\ $$$$ \\ $$$$\mathcal{O}{bserve}\:{the}\:\mathcal{PATTERN}: \\ $$$${LHS} \\ $$$$\left(\mathrm{1}\right):{Sum}\:{of}\:{all}\:{symbols}. \\ $$$$\left(\mathrm{2}\right):\:{Sum}\:{of}\:\underset{−} {{squares}\:{of}\:{all}\:{the}\:{symbols}} \\ $$$$\left(\mathrm{3}\right):{Minus}\:{sum}\:{of}\:{products}\:\:{of}\:\:\:\underset{−} {\:\:{possible}\:{combinations}\:{of}\:{two}} \\ $$$${RHS} \\ $$$$\left(\mathrm{4}\right):{Sum}\:{of}\:{cubes}\:{of}\:{all}\:{the}\:{symbols}. \\ $$$$\left(\mathrm{5}\right):{Minus}\:{three}\:{times}\:{the}\:{sum}\:{o}\underset{−} {{f}\:\:{possible}\:{combinations}\:{of}\:{three}\:\:\:} \\ $$$$\mathcal{C}{oppying}\:{the}\:\mathcal{PATTERN}: \\ $$$$\left(\underset{\left(\mathrm{1}\right)} {\overset{−−−−{sum}−−−−} {{a}_{\mathrm{1}} +{a}_{\mathrm{2}} +…+{a}_{{n}} }}\right)\left(\underset{\left(\mathrm{2}\right)} {\overset{−{sum}\:{of}\:{squares}−} {{a}_{\mathrm{1}} ^{\:\mathrm{2}} +{a}_{\mathrm{2}} ^{\:\mathrm{2}} +…+{a}_{{n}} ^{\:\mathrm{2}} }}\underset{\left(\mathrm{3}\right)} {\overset{{possible}\:{combination}\:{of}\:{two}\:{symbols}} {−{a}_{\mathrm{1}} {a}_{\mathrm{2}} −{a}_{\mathrm{1}} {a}_{\mathrm{2}} −…}}……..\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:=\underset{\left(\mathrm{4}\right)} {\overset{−−{sum}\:{of}\:{cubes}−−} {{a}_{\mathrm{1}} ^{\:\mathrm{3}} +{a}_{\mathrm{2}} ^{\:\mathrm{3}} +…+{a}_{{n}} ^{\:\mathrm{3}} }}\underset{\left(\mathrm{5}\right)} {\overset{−−{possible}\:{combination}\:{of}\:{three}\:{symbols}−−} {−\mathrm{3}{a}_{\mathrm{1}} {a}_{\mathrm{2}} {a}_{\mathrm{3}} −\mathrm{3}{a}_{\mathrm{1}} {a}_{\mathrm{2}} {a}_{\mathrm{4}} −….}}…………… \\ $$