(5a+4b)的平方-(5a-b)(5a+b)=?
(5a+4b)^2-(5a-b)(5a+b)
=(25a^2+16b^2+40ab)-(25a^2-b^2)
=17b^2+40ab
2x^2-7x-15=(2x-3)(x+5)
你得的是(2x+5)(x-3)
2x^2-7x-15=(2x+3)(x-5)
7x=2x+15
7x=2x+15
7x-2x=15
5x=15
x=3