if 1-cos b = 2/5 , prove that 25 sin b [ sin b - cos b ] = 4
1-cosb =2/5
cosb =1-2/5
cosb=3/5
sin²b =1-cos²b
=1-9/25
= 16/25
sinb= 4/5
25sinb(sinb-cosb) =25 ×4/5 × (4/5-3/5)
=5×4×1/5
=4