a) Ta có: \(\left(3x-5\right)\left(3x+5\right)-\left(3x-8\right)^2\)
\(=9x^2-25-9x^2+48x-64\)
\(=48x-89\)
b) Ta có: \(\left(7x-5\right)\left(7x+5\right)-\left(8x+5\right)^2\)
\(=49x^2-25-64x^2-80x-25\)
\(=-15x^2-80x-50\)
c) Ta có: \(\left(8x-5\right)\left(5x-4\right)-\left(5x-5\right)\left(5x+5\right)\)
\(=40x^2-32x-25x+20-25x^2+25\)
\(=15x^2-57x+45\)
d) Ta có: \(\left(6x-7y\right)\left(6x+7y\right)-\left(8x-5y\right)^2\)
\(=36x^2-49y^2-64x^2+80xy-25y^2\)
\(=-28x^2+80xy-74y^2\)
e) Ta có: \(\left(2x-3\right)\left(2x+3\right)-\left(5-7x\right)^2\)
\(=4x^2-9-25+70x-49x^2\)
\(=-45x^2+70x-34\)
f) Ta có: \(\left(5x-6y\right)\left(4x-2y+5\right)+\left(3x+7y\right)^2\)
\(=20x^2-10xy+25x-24xy+12y^2-30y+9x^2+42xy+49y^2\)
\(=29x^2+25x+8xy-30y+61y^2\)
