①(2a+3b)² =4a²+12ab+9b²②(-xy+5)² =x²y²-10xy+25③(x+3)(x-3)(x²+9) =(x²-9)(x²+9)=x^4-81④51²+49²=(50+1)²+(50-1)²=2×50²+2×1²...