Question

a6+a9+a12+a15=20,  S20=?

Answer 1

а6 + а9 + а12 + a15 = 20; a1+5*d+a1+8*d+a1+11*d+a1+14*d=20; 4*a1+38*d=20; S20=0,5*(2*a1+19*d)*20; 
2*a1+19*d=10; S20=0,5*10*20=100;