In figure (A), the effective spring constant, K= k1+k2
In figure (B), the effective spring constant 1K=1k1+1k2 or K=k1k2(k1+k2)
In figure (C), the effective spring constant, K=k+k=2k
In figure (D), let the effective spring constant be K.
If mass m be pulled down by distnace y, the springs A and B will be stretched by y′(=ycos45∘ )
Total restoring force is F=Ky= FAcos45∘+Fbcos45∘=2(ky′)cos45∘=2k(ycos45∘)cos45∘ =ky
∴ K=k; and time period T=2π√mK