915.
a) (11c²+a³)(-a³+11c²)=(11c²+a³)(11c²-a³)=(11c²)² - (a³)²=
=121c⁴ - a⁶
б) (0.8x+y⁴)(-0.8x-y⁴)=(0.8x+y⁴)(-(0.8x+y⁴))= -(0.8x+y⁴)²=
=-(0.64x²+1.6xy⁴+y⁸)= -0.64x² -1.6xy⁴ -y⁸
в) (0.3c-0.2d)(0.2d-0.3c)=(0.3c-0.2d)(-(0.3c-0.2d))=-(0.3c-0.2d)²=
= -(0.09c² - 0.12cd+0.04d²)= -0.09c² +0.12cd - 0.04d²
г) (6x³ -4x)(-6x³-4x)=(6x³-4x)(-(6x³+4x))= -((6x³)² - (4x)²)=
= -(36x⁶ - 16x²) = 16x² - 36x⁶