Factorise: 25a2−4b2+28bc−49c2
25a2−4b2+28bc−49c2=25a2−[4b2+28bc−49c2] {∵a2−2ab+b2=(a−b)2}=(5a)2−[(2b)2−2×2b×7c+(7c)2]=(5a)2−(2b−7c)2 {a2−b2=(a+b)(a−b)}=(5a+2b−7c)(5a−2b+7c)
Factorise 25a2−4b2+28bc−49c2