問3題數學題!!

1.18(m-5)²-24(m-5)
=6(m-5){3(m-5)-4}
=6(m-5)(3m-19)//
2)1/36r²-3rs+81s²
=(r/6)²-3rs+(9s)²
=(r/6)²-2(r/6)(9s)+(9s)²---------------(since 3rs=2(r/6)(9s))
=(r/6-9s)²//---------------use the formula a^2-2ab+b^2=(a-b)^2
3.49x²-21x+6y-4y²
=49x²-4y² -21x+6y
=(7x)²-(2y)²-3(7x-2y)
=(7x+2y)(7x-2y)-3(7x-2y)
=(7x-2y)(7x+2y-3)//