(a) a+b+c=5 ......(1) ab+bc+ca=10 .....(2) a+b+c=5 Squaring on both sides - (a+b+c)2=52 Putting the value from Eqn. (2) a2+b2+c2+2(ab+bc+ca)=25 a2+b2+c2=25−20 a2+b2+c2=25.......(3) Putting the value from Eqn. (1), (2) and (3) a3+b3+c3−3abc=(a+b+c)[a2+b2+c2−(ab+bc+ca)] =5[5−10] =5×(−5)=−25.