3.
a) (x+5)²-5x(2-x)=x²+10x+25-10x-5x²=25-4x²
б) 5(a+b)²-10ab=5(a²+2ab+b²)-10ab=5a²+10ab+5b²-10ab=5a²+5b²
в) (c-2)(c+3)-(c-1)²=c²+3c-2c-6-(c²-2c+1)=c²+c-6-c²+2c-1=3c-7
г) 3(2-x)²-(2x²+x-5)(x²-2)+(x²+4)(4-x²)=3(4-4x+x²)-(2x⁴-4x²+x³-2x-5x²+10)+16-x⁴=12-12x+3x²-2x⁴-x³+9x²+2x-10+16-x⁴=-5x⁴-x³+12x²-10x+2
4.
(2-x)(3+x)+(4-x)(5-x)=6+2x-3x-x²+20-4x-5x+x²=-10x+26=-10(3/20)+26=-3/10+26=-0,3+26=25,7