Răspuns :

[tex]\bf\ b.\ \it\ =3(1+2+3+\ ...\ +30)=3\cdot\dfrac{30\cdot31}{2}=3\cdot15\cdot31=[/tex]

[tex]\bf c.\ \it =2+5+8+11+14+17+20+23+26+29 =\\ \\ (2+29)+(5+26)+(8+23)+(11+20)+(14+17) =31+31+31+31+31=[/tex]