Đáp án:
Giải thích các bước giải:
Ta có :A=12/5.2.5+12/5.5.8+. . .+12/5.17.20
A =`1/5` x `12/2.5`+`1/5` x `12/5.8`+. . .+`1/5` x `12/17.20`
A = `1/5` x(`12/2.5`+`12/5.8`+. . .+`12/17.20` )
A = `1/5` x 4 x(`3/2.5`+`3/5.8`+. . .+`3/17.20`)
A = `4/5` x ( `1/2`-`1/5`+`1/5`-`1/8`+. . .+`1/17`-`1/20`)
A = `4/5` x ( `1/2`-`1/20`)
A = `4/5` x `9/20`
A =`9/25`