(1-1/4)×(1-1/9)×(1-1/16)×(1-1/25)×(1-1/36)×(1-1/49)
=1×3/2²×2×4/3²×3×5/4²×4×6/5²×5×7/6²×6×8/7²
=1/2×8/7
=4/7