suku ke -70 dari barisan 4,10,18,28,.....adalah...

un = a+ (n-1) b
 u 70 = 4+ (70-1) x 6
           4 + 69 x 6
              4 + 414 =418 

4,10,18,28,40,54,70,88,108,130,154,180,208,2 38,270,304,340,378,418,460,504,550,598,648,700,754,810,868,928,990,1.054,1.120,1.188,1.258,1.330,1.404,1.480,1.558,1.638,1.720,1.804,1.890,1.978,2.068,2.160,2.254,2.350,2.448,2.548,2.650,2.754,2.806,2.968,3.078,3.190,3.304,3.420,3.358,3.658,3.780,3.904,4.030,4.158,4.288,4. 420,4.554,4.690,4.828,4.968,5.110