13)a-9/a+6√a+9=(√a-3)(√a+9)/a+3√a+3√a+9=(√a-3)(√a+3)/(√a+3)²=√a-3/√a+3
14)x⁴-6x²+5=0
x⁴-6x²+5+4=4
x⁴-6x²+9=4
x⁴-3x²-3x²+9=4
x²(x²-3)-3(x²-3)=4
(x²-3)²=9 x²-3=2 x²-3=-2
x₁=√5 x₂=1 x₃=-1
15)√2-1/√2+1+√2+1/√2-1=(√2-1)(√2-1)/(√2-1)(√2+1)+(√2+1)(√2+1)/(√2-1)(√2+1)=(√2-1)²+(√2+1)²/(√2-1)(√2+1)=2-2√2+1+2+2√2+1/(√2-1)(√2+1)=6/2-1=6