简算①925-（337+0.4）②1.8×14+2.2×25%③11×3+13×5+15×7+…+117×19+119×21．

Question

简算
①9 (2)/(5) -（3 (3)/(7) +0.4）
②1.8× (1)/(4) +2.2×25%
③ (1)/(1×3) +(1)/(3×5) +(1)/(5×7) +…+(1)/(17×19) +(1)/(19×21) ．

Answer 1

①9 (2)/(5) -（3 (3)/(7) +0.4），
=9 (2)/(5) -3 (3)/(7) - (2)/(5) ，
=9 (2)/(5) - (2)/(5) -3 (3)/(7) ，
=9-3 (3)/(7) ，
=5 (4)/(7) ；

②1.8× (1)/(4) +2.2×25%，
=1.8×0.25+2.2×0.25，
=（1.8+2.2）×0.25，
=4×0.25，
=1；

③ (1)/(1×3) +(1)/(3×5) +(1)/(5×7) +…+(1)/(17×19) +(1)/(19×21) ，
= (1)/(2) ×（1- (1)/(3) ）+ (1)/(2) ×（ (1)/(3) - (1)/(5) ）+…+ (1)/(2) ×（ (1)/(17) - (1)/(19) ）+ (1)/(2) ×（ (1)/(19) - (1)/(21) ），
= (1)/(2) ×（1- (1)/(3) + (1)/(3) - (1)/(5) +…+ (1)/(17) - (1)/(19) + (1)/(19) - (1)/(21) ），
= (1)/(2) ×（1- (1)/(21) ），
= (1)/(2) × (20)/(21) ，
= (10)/(21) ．