1／1×3＋1／3×5＋1／5×7······﹢1／47×49
=1/2×(1-1/3)+1/2×(1/3-1/5)+...+1/2×(1/47-1/49)
=1/2×(1-1/'3+1/3-1/5+1/5-1/7+...+1/47-1/49)
=1/2×(1-1/49)
=24/49