Question

(4x+5x) (4x-5y)-(3x2y)(3x-2y)

Answer 1

given,
(4x + 5x)(4x - 5y) - (3x + 2y)(3x - 2y)
first , see the first term ( i mean 4x)
multiply it with (4x - 5y)
u will get,
16x² - 20xy
like wise multiply +5x with (4x - 5y)
u will get,
20 x²- 25 xy
now (4x + 5x)(4x-5y) = 16x²-20xy+20x²-25xy
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now,u have
-(3x +2y)(3x-2y)
similarly how u did above ,
multiply -3x with ( 3x-2y)
u will get,
-9x²+6xy
now,
u should take -2y and mul with (3x-2y)
u will get,
-6xy + 4y²
now-(3x+2y)(3x-2y) = -9x²+6xy-6xy+4y²
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now, totally,

(4x+5x) (4x-5y)-(3x2y)(3x-2y)=16x²-20xy+20x²-25xy-9x²+6xy-6xy+4y²
 arrange in an easy way to solve,
                                             = 16x²+20x²-9x²+4y²-20xy-25xy+6xy-6xy
                                             = 37x²+4y²-45xy
here, +6xy and - 6xy gets cancelled
u need to concentrate on symbols :
(i) plus×plus = plus
(ii)plus ×minus = minus
(iii) minus × minus = plus
(iv) minus ×plus = minus