(1) h′=20−10t [m/s]h' = 20 - 10t \, [m/s]h′=20−10t[m/s] (2) h′=0h' = 0h′=0 のとき 20−10t=020 - 10t = 020−10t=0 t=2t = 2t=2 ２秒後 (3) h=0h = 0h=0 のとき 25+20t−5t2=025 + 20t - 5t^2 = 025+20t−5t2=0 5(5−t)(1+t)=05(5-t)(1+t) = 05(5−t)(1+t)=0 t=5,−1t = 5, -1t=5,−1 t=−1t = -1t=−1 は不適なので t=5t = 5t=5 このとき h′=20−50=−30h' = 20 - 50 = -30h′=20−50=−30 −30 [m/s]-30 \, [m/s]−30[m/s]