This is an implementation of quadratic fit for WP-34s. If only 3 points are entered, this may be used for quadratic interpolation.
Similar function was implemented in HP30b, if you missed it, here it is.

// (C) 2013 Andrew Nikitin
// Fit quadratic polynomial to empirical data
// program operates in stack size 4
// uses regJ and regK to accumulate [SIGMA]x^3 and [SIGMA]x^4
// uses stat registers to accumulate other sums
// stores coefficients of quadrature fit in regA, regB, regC

// Initialize:
// XEQ'QF'
// Clears sums, but not coefficients

// Enter point:
// y ENTER x R/S (or [A])
// Result:
// number of points entered so far

// Calculate coefficients (needs at least 3 points):
// [B]
// Result:
// a=regA=regZ
// b=regB=regY
// c=regC=regX
// (order compatible with SLVQ)

// Evaluate Ax[^2]+Bx+C polynomial:
// x [C]
// Result:
// Ax[^2]+Bx+C

LBL'QF'
SSIZE4
CL[SIGMA]
CLx
STO J
STO K
XEQ 00
STOP
XEQ A
BACK 002
// Accumulate sums, including [SIGMA]x^3 and [SIGMA]x^4
LBL A
[SIGMA]+
RCL L
ENTER
x^3
STO+ J
*
STO+ K
LBL 00
CLx
n[SIGMA]
RTN

// delete point
LBL 65 // XEQ -
[SIGMA]-
RCL L
ENTER
x^3
STO- J
*
STO- K
GTO 00

// Fit quadratic
LBL B
LocR 016
RCL K
STO .03
RCL J
STO .02
STO .06
# 002
n[SIGMA]
x<=? Y
ERR 15
STO .07
[SIGMA]x
STO .04
STO .08
[SIGMA]x[^2]
STO .01
STO .05
STO .09
[SIGMA]y
STO .12
[SIGMA]xy
STO .11
[SIGMA]x[^2]y
STO .10
1
1
3
.
0
3
0
3
ENTER
1
2
2
.
0
3
0
1
# 125
LINEQS
RCL .15
STO A
RCL .14
STO B
RCL .13
STO C
RTN

// Evaluate Ax^2+Bx+C polynomial at regX
LBL C
ENTER
RCL* A
RCL+ B
*
RCL+ C
RTN

END