Diketahui luas tabung tanpa tutup 7986 cm². Jika tinggi tabung 50 cm, diameter alas tabung adalah...(π = 22/7) A. 21 cm B. 28 cm C. 35 cm D. 42 cm

7.986 = π . r . (r + 2t)
7.986 = 22/7 . r . (r + 2 . 50)
r² + 100r = 7.986 . 7/22
r² + 100r = 2.541
r² + 100r - 2.541 = 0
(r + 121) (r - 21) = 0
r + 121 = 0
= -121
r - 21 = 0
= 21 ←←

d = 21 x 2 = 42


(D)